# Illusions

## You’re Getting Warmer! [W/Video]

The Little Shop of Physics has developed a series of videos called Flash Science, which show some exciting experiments that can be done with everyday items to demonstrate physics principles in a unique way. All of these experiments have been designed to be done by trained adults using proper safety equipment.

### Heat

In physics, heat is something you do; it’s a verb. It is defined as the thermal (non-mechanical) transfer of energy. When you heat an object, you transfer energy to it, which can raise its temperature or even cause a phase change. Traditionally, three sources of heat transfer are cited: convection, conduction and radiation.

Radiation is the transfer of thermal energy using electromangetic waves, which includes visible light, infrared radiation, ultraviolet radiation, x-rays and microwaves and radio waves. A camera flash is designed to give off a whole lot of visible light in a short amount of time. The black ink in the newspaper absorbs this radiation and increases in temperature, while the blank paper reflects the light and does not warm up nearly as much.

### Conduction and Convection

When a flame is held underneath a balloon, it’s no surprise that the balloon pops. The flame is at a high enough temperature to heat and melt (or even burn) the balloon, and the air under pressure inside quickly escapes. However, when the balloon is filled with water, the flame no longer pops it. The balloon is very thin, and the thermal energy quickly gets conducted to the water on the inside. The water has a very high heat capacity, so it takes a large amount of energy to increase the temperature of the water.

The water is also effective transferring the thermal energy away from the flame. The water will undergo convection; the warm water by the flame will move upwards, and be replaced by colder water coming in from the sides. Also, since water evaporates at 100°C, liquid water has a limit on how high of a temperature it can reach.

### Evaporation

Evaporation is an extremely important and sometimes overlooked form of thermal energy transfer. Evaporative cooling is the mechanism behind human sweating, and the energy stored in evaporated water is extremely important in the Earth’s weather system.

In this video the flame hounds are soaked in a mixture of rubbing alcohol and water. While the alcohol burns, and releases thermal energy, the water evaporates and takes much of that thermal energy away from the flame hounds, so that it does not burn!

If you’re careful, you can even hold flaming bubbles in your hands!

### Plasmas

Running electricity through the graphite pencil-lead causes the tip to get extremely hot, so hot that the graphite vaporizes and the vapor ionizes. These hot ions are used to cut aluminum foil, similar to how a plasma cutter or arc cutter works.

### Erasing With Heat

Some erasable pens use thermochromic ink, which changes colors from dark to light when it is heated. When the ink is cooled (such as through the evaporation of a liquid), the ink becomes dark again. With this ink, you can erase and re-write messages over and over again!

## Simple Model of How the Sky Changes Color at Sunset

The Sunset Egg is a fun and engaging demonstration on the science of light. The egg is made of “opalescent” glass. This refers to the way it looks different at different viewing angles (similar to the gem’s properties).

An oil-polished Sunset Egg, lit from below, shows both blue sky and yellow sun.

To use the egg hold it in one hand and close your hand around it. The egg will appear blue. Now hold it up to a source of white light, such as overhead lights. The egg will appear yellow. What’s going on? How can this be used to explain the blue sky and the yellow sunset?

The Sunset Egg responds differently to light based on its wavelength (The sky does the same thing). This process is called Rayleigh Scattering and when light is scattered, the shorter wavelengths are scattered more often.

When light hits the egg, more blue light is scattered than red and yellow, thus the egg usually looks blue. But the light that passes through the egg has had its blue light scattered away. The remaining light is yellow and red. Looking through the long end of the egg or using multiple eggs can also increase the effect.

The yellow sunset and the daytime blue sky are caused by different path lengths through the atmosphere. The longer the path, the less blue light remains.

But how can the egg help explain the sky?

During the day the light we see in the sky comes from light being scattered by air molecules (mostly oxygen and nitrogen). Since shorter wavelengths get scattered more often, the blue light is more frequently scattered. During sunset, the light has to pass through a more of the sky and that journey causes the blue light to get scattered out sideways on its way. The result is yellow and orange sunsets.

Sunlight that has lost its blue looks yellow.

This is easily seen in the egg. The light scattered sideways is blue, but the light traveling all the way through is yellow. In the case of the sky, the light is being scattered on air molecules, mostly oxygen and nitrogen, but also dust and other particulates. In the case of the egg, the light is being scattered on fine dye particles inside of the glass.

The blue sky effect clearly shown on the top half of the egg. Note that the scattered blue light (moving to your eye) is perpendicular to the incoming light. The light that passes all the way through the egg is very yellow.

The egg behaves like a little piece of the sky, and it looks like one for the correct reason – scattering. When the light passes through a small bit of it, the egg or sky looks blue, but when light passes through a lot of it, whether it is the egg or the sky, it looks yellow.

A similar blue sky effect can be achieved by using an aquarium full of water with a little coffee creamer. When light passes through the aquarium it gets scattered by the tiny coffee creamer particles. But blue light gets scattered more frequently, making the aquarium look blue over all.

The light that passes through the aquarium has less blue in it and so it looks yellow. This causes a sunset effect. It is not just an effect however; this is the real cause.

When you first get the egg, it can be used immediately for these experiments. However, it might have a sheen of white dust. This can be washed off somewhat, but it is helpful to wipe cooking oil over it and then dry it off with a paper towel. This will give the egg a smooth surface and improve the demonstrations that follow.

Cooking oil provides polish for a dull egg.

The reason the cooking oil smooths out the opalescent glass egg is because oil and glass have nearly the same index of refraction; they bend light by the same amount.

The Sunset Egg may be the best science gift ever because it is so much fun and can teach us so much.

## Sunset Egg

In Stock SKU: P2-1000
\$5.00

## Sunset Egg 6 Pack

In Stock SKU: P2-1001
\$25.00

## Sidewalk Art of Julian Beever

The great artwork of Julian Beever is a side bar to the Arbor Scientific CoolStuff Newsletter on Science in Art. If you find this as fascinating as we did you might want to see the Shadows in Science and Art article.

### How it’s done…..

From the front looking down the sidewalk with the Artist standing at the end of the image.

Looking from the side of the walk gives a completely different perspective on the actual image.

## 3D Mural Art of Artist Eric Grohhe

### Using Shadows in Real World Art

The first image here is a “before picture of the subject building. Your standard stucco and concrete look.

This image shows the Artist preparing his canvas.

Artist Eric Grohe starts to take on a 3 dimensional appearance. Eric in his element, 30′ off the ground. He does most of the artwork by himself & researches, paints and designs each project from scratch. His wife Kathy acts as project manager.

Here is the finished product…

Here are so more examples of his work. Great American Crossroad – Bucyrus, Ohio –
Before…

…and After!

Liberty Remembers! Before…

…and after!

How to dress up a shopping mall. Niagara, NY before…

After

Close-up view

Inside a brewery…

…and after!

For more information on Eric Grohe and his mural work go to http://www.ericgrohemurals.com

## Shadows in Science and Art

Shadows are ubiquitous, but often go unnoticed. Shadows are important historically, for they provided early evidence that light travels in straight lines. Humans constantly, but unconsciously, use shadows to judge the shape of objects in their environment. Because shadows reveal much about an object’s extension in space, they are often used to heighten the illusion of depth in a painting.

Lets look at some exploratory activities using shadows that may be used to introduce geometrical optics and demonstrate applications of shadows in perception and the visual arts.

In a darkened room, use a point source of light to form the shadow of a small object, such as a small box or ball, on a screen. An LED flashlight with a single bulb or automobile tail light bulb serves as a good approximation to a point source. When a single point source is used to illuminate an object, two distinct regions should be observed. In one region, light from the source is completely blocked. This region is referred to as the umbra. Outside the umbra, light is not affected by the object.

When a second point source is introduced as shown in the figure below, you should observe two types of shadowing. Once again there is the umbra, a region on the screen that is in complete darkness. In the umbra, light from the two sources is blocked. Outside the umbra is the penumbra where light from one bulb reaches the screen but light from the other bulb does not.

Now replace the two point sources with what is referred to as an extended source. An extended source may be thought of as a very large number of point sources. A frosted light bulb and a fluorescent tube are examples of extended sources. Describe the shadows produced by an extended source.

To understand the shadows produced by an extended source, it may be useful to remember that each point on the surface of the bulb, acting as a point source, produces its own shadow. When all these point source shadows are superimposed, there will be total darkness (umbra) surrounded by a lighter region (penumbra) where only some of the individual shadows overlap.

During eclipses of Sun, the moon intervenes and casts a shadow on the earth. Observers in the umbra see a total eclipse, while observers in the penumbra see only a partial eclipse.

2001 Solar Eclipse Composite by Wendy Carlos, Williams College.

The earth is approximately 93 million miles from the sun. At this great distance, can the sun be considered to be a point source of light? A simple experiment should provide an answer.

The next time you are out on a sunny day examine the shadow produced by a pole, a leaf, your hand, or virtually any other object. How will this observation help you answer the question?

Humans constantly, but unconsciously, use shadows to judge the shape and location of objects in their environment. In doing so, we all rely on the default assumption that sources of light are overhead. We live in a world where light almost always comes from above. Have you noticed how children never cease to be delighted by the effects produced by illuminating the face with a light from below? Why does this always bring chuckles? Humans are simply not used to seeing the shadows formed by a light source located beneath the face.

Sometimes this hard-wired assumption regarding light placement can lead to incorrect conclusions regarding the nature of an object. For example, in is this photograph we see large indentations among an array of rivets on the hull of a ship. This percept is based on the nature of the shadows and the assumption that the light source is overhead. When the photo is inverted, things change dramatically! Rivets become divots, and vice versa.

Image Right side up

Same image turned upside down

While students always enjoy this demonstration, some may ask “who cares.” You may wish to point out to them that astronauts landing on the moon care a great deal about the actual nature of the lunar surface.

In the figure below, we see a crater. However, when the photo is turned upside down (right), the shadows suggest otherwise. We now see a hill.

Crater Image

Crater Image Upside Down

In this photograph from “Walter Wick’s Optical Tricks,” we see a number of pieces of wood on a woodworker’s bench. The odd-shaped pieces of wood are illuminated from above as we can see from the shadows. When the photo is turned upside down, and the shadows shift, we see something entirely different! Do you see the deer surrounded by branches and leaves?

Deer demo of a page from Walter Wick’s Optical Tricks.

We no longer carry “Walter Wick’s Optical Tricks” book.

Because shadows reveal much about an object’s extension in space, they are one of an artist’s most potent depth cues.  Notice how a circle becomes a sphere with the addition of shadow and shading. Where is the source of light in this drawing?

In this rather simple sketch of an elephant by Rembrandt, the sense of depth and solidity is due in large part to the adroit use of shadows.

Rembrandt van Rijn, An Elephant, black chalk and charcoal, around 1637

In Escher’s “Drawing Hands,” shadow and shading are used to create a sense of three-dimensionality. The hands seem to pop right off the piece of paper.

Escher, Maurits Cornelis: Drawing Hands 1948
In paintings such as “An Experiment on a Bird in the Air Pump” by Joseph Wright, the use of strong contrasts of light and dark may be used to discuss the nature and location of the light source as well as the inverse square law. The rather sharp shadows suggest a point source such as a candle. Notice too that even though the light source cannot be seen, its location can be inferred. And perhaps most importantly, the shadows establish the mood of the painting.

An Experiment on a Bird in an Air Pump by Joseph Wright of Derby, 1768
The scene in Edward Hopper’s “The Night Hawks” is totally devoid of harsh shadows. Why? When this work was created, fluorescent lights had become commonplace. The uniform lighting produced by a collection of extended sources does not produce sharp shadows. The result: a mood of detachment and loneliness.

Nighthawks (1942) by Edward Hopper.

Copyright NoticeThese images are of a drawing, painting, print, or other two-dimensional work of art, and the copyright for it is most likely owned by either the artist who produced the image, the person who commissioned the work, or the heirs thereof. It is believed that the use of low-resolution images of works of art; for critical commentary on, the work in question, the artistic genre or technique of the work of art, qualifies as fair use under United States copyright law.

4. Turning Things Inside Out with Shadows
A demonstration of the power of shadows that never ceases to amaze students involves reversed three-dimensional figures. By manipulating the light striking a concave object it is possible to make it appear convex.

To observe this reversal, cut an L-shaped piece of paper consisting of three segments about two inches square (see figure). Fold at the two lines joining the squares and join them with transparent tape to make half a cube.

With one eye closed, hold the concave corner at arm’s length and orientate it so that it appears to be convex. That is, at some orientation, the concave cube corner will appear to reverse itself! Amazing! Once you have achieved reversal of the corner, rotate it in your hand and notice that the cube appears to turn in the opposite direction.

An inexpensive plastic mask may be used to illustrate the same effect. With one eye closed, the concave side (backside) of the mask appears convex. Furthermore, the face seems to follow you as you move from side to side! Working in concert with the shadow cues is our expectation to see a convex face. We have rarely seen a convex face, so we tend to see what we believe.

Einstein Alive! You have to see it to believe it!

Shine a source of light at the back of this mask and look at the concave side. The mask appears to reverse, as if Einstein is looking at you! Move back and forth, and his face turns to follow you. Move up and down, and he nods his head.
See Einstein Alive

We just received these amazing photos that fit perfectly with our topic, so we thought we would share them with you. Click the link to see the whole series…

## Atmospheric Optics: She comes in colors…

The sky offers a wide variety of stunning optical effects. A source of inspiration for poets and songwriters alike, these atmospheric phenomena include red sunsets, rainbows, mirages, halos, glories, and coronas. These effects are the result of the interaction of light from the sun or moon with the gases in the atmosphere, clouds, ice crystals, smoke, dust and other airborne particulates. Some of these phenomena can be seen almost every day; others occur less frequently. In this issue of CoolStuff we will examine examples of atmospheric optical phenomena and how they may be demonstrated in the classroom.

The sky is the daily bread of the eyes.
– Ralph Waldo Emerson

She comes in colors everywhere;
She combs her hair
She’s like a rainbow
Coming colors in the air
Oh, everywhere
She comes in colors…
She’s like a rainbow

– Mick Jagger / Keith Richards

Concept:
A rainbow is a multicolored, circular band of light. The display of colors is due to refraction and internal reflection occurring in raindrops or other droplets of water.

Direct a fine spray from a garden hose in a direction away from the sun. How far away do you estimate the rainbow to be? If you do this experiment with a group of people, does everyone see the same rainbow? Do you see your shadow? Where is it located in relation to the rainbow? If you want to explore further, stand on a ladder while producing your rainbow. Describe the rainbow you see now.
In a darkened room, place a clear (the clearer, the better) plastic box approximately three-quarters full of water on the stage of an overhead projector. (Note: These boxes are the type often used to store shoes.) Cover or remove the projector’s top lens so that no light is projected into the room. Arrange the water-filled box so that students can see both of the rainbows formed (a rainbow is produced by each long side of the box.) Examine the array of colors produced by the water-filled plastic box. Are the rainbow colors in the same order as in a naturally-occurring rainbow?

Shine light from the bright flashlight or a slide projector through a central hole in a piece of white cardboard. If a water-filled flask is illuminated with the light passing through the hole (see figure) a faint rainbow will appear on the cardboard. It has the shape of a closed circle and its angular distance is about 42 degrees, with red on the outside, as in a naturally occurring rainbow. You will need a completely dark room since the rainbow formed is quite faint.
Spectrum demonstration:
Discussions on rainbows and the optics of the sky always lead to the topic of the electromagnetic spectrum.

## Spectrum Analysis Classroom Set

In Stock SKU: P2-9501
\$489.00

Another great classroom tool is the Giant Prism. Use it on your overhead projector to project a large class-size rainbow!

## Giant Prism

In Stock SKU: 33-0230
\$27.00

Hiroto Ashikaga; Tottori Technical High School, Syozan 111, Tottori 689-1103 Japan

Making Your Own Rainbow IV: Tiny glass beads, such as those used by your local highway department to make highway signs and street markings highly reflective, may be used to produce rainbows like those seen in the center photo below. The beads, behaving like raindrops, work in concert to form a rainbow.

Most highway and public works departments will gladly give you a container of glass beads. Once you have obtained the beads, cover a piece of black foam core or poster board with a thin layer of spray glue. Now sprinkle the glass beads over the black surface until the surface is completely covered with beads. When a point source of light, such as a Maglite with reflector removed, is used to illuminate the beads, the beads will form a circular rainbow that seems to hover above the cardboard.

Concept: Blue light interacting with molecules in the atmosphere is absorbed and reradiated in all directions. Blue light is scattered much more efficiently than light with longer wavelengths, for example, red and green. As a result of scattering, the sky looks blue no matter where we look. By contrast, to an observer on the moon, the lunar sky appears black because there is no atmosphere to scatter light.

During sunrise and sunset, the distance that light travels from the Sun to an observer on Earth is at its greatest. This means that a large amount of blue light and some green light is scattered. Since white sunlight may be thought of as consisting primarily of blue, green and red light, the blue/green deficient light that we see coming directly from the sun appears red.

Blue Sky/Sunset Simulation I: One of the most frequently asked questions is “why is the sky blue?” Using very simple equipment, you can demonstrate and explain the phenomenon to your students. Add a few drops of milk or a few grains of powdered milk to water in a beaker or fish tank and stir. The milk particles serve as scatterers just as air molecules do in the atmosphere. When light from a light bulb or slide projector passes through the liquid, scattered blue light may be seen throughout the container.

Shine light from a light bulb or slide projector through the liquid and observe the color of the transmitted light. With much of the blue light removed from the incident white light by scattering, only the orange-red portion of the spectrum remains. When viewed head on through the liquid, the transmitted light actually looks like a setting sun!

If you are using a slide projector and fish tank for the simulation, you may wish to carefully rotate the tank as it is illuminated. Allowing light to first pass through the narrow width, then through the length of the tank, allows students to observe how the color of the sun changes from a yellow-orange to an orange-red as it moves from its noon day position to the horizon.

Blue Sky/Sunset Simulation II: A second method of demonstrating why the sky is blue and the sunset red requires the use of two common chemical substances: dilute sulfuric acid (H2SO4) and sodium thiosulfate (Na2S2O3), hypo used in photography to fix developed films. (Caution: be careful when handling the acid.) First mix three teaspoons of thiosulfate with one liter of water. To this solution add ten to twelve drops of acid. After a few seconds, the solution will take on a bluish tint. With time the color will become more intense, then fainter. After a few minutes the liquid will turn white.

These changes are due to the scattering of white light from tiny grains of sulfur which gradually grow in size as the reaction progresses. Initially, the grains are very small and serve as scattering centers for short wavelengths of light, hence the blue color. Eventually the particles become so large that they scatter all wavelengths of visible light with equal intensity. This accounts for the final milky appearance of the liquid.

Note that a cardboard mask blocks the light not passing through the beaker.

Scattering from particles whose dimensions are much less than the wavelength of light is known as Rayleigh (pron. ray-lee) scattering. Rayleigh scattering is responsible for the blue appearance of the Earth’s sky. The non-preferential scattering by larger particles is known as Mie (pron.me) Scattering and is responsible to the white color of clouds.

A beautiful setting sun effect can be achieved by placing a beaker containing the H2SO4 – Na2S2O3 solution on the stage of an overhead projector (see image left). First mix three teaspoons of thiosulfate with one liter of water. To this solution add ten to twelve drops of acid. (Caution: be careful when handling the acid.)

A mirror is used to project the light passing through the beaker onto a screen. As the sulfur particles grow in size, the scattered blue light will become more intense while the light reaching the screen will change from white, to yellow, to orange and finally to a deep red.

The Color of Clouds

Concept: Clouds consist of water droplets and ice crystals that are significantly larger than the wavelengths of visible light. Unlike the smaller gas molecules that make up the Earth’s atmosphere, these larger particles scatter all colors more or less equally.

Looking at a cloud, an observer will, in most cases, receive all wavelengths of light and perceive it as white. However, a cloud’s actual appearance is governed by color of illuminating light, cloud thickness, shadowing by other clouds, age of the cloud, and the brightness of surrounding sky and clouds. Thicker clouds transmit little light and hence may appear darker. Larger droplets in older clouds scatter less and absorb more light than smaller drops and therefore appear darker.
The Whitest Cloud Around:
What we identify as white is simply the brightest gray in sight. A light gray cloud on a bright white background will look much darker than the same cloud on a dark or black background, in which case it might look white and bright. To demonstrate this, obtain a variety of paper samples, each of which appears to be white in isolation. Place them side by side, or cut them so that they can be nested on top of one another, for comparison. Usually only one will be perceived as white; the other samples will appear gray by comparison, as it is with clouds.
Concept:
A halo is an optical phenomenon due to reflection and refraction of sunlight or moonlight in atmospheric hexagonal ice-crystals. Halos appear as bright rings around the sun or moon. Although they are more common in cold weather, halo-producing cirrus clouds can be present in warm weather. Colored halos are formed by refraction in the crystal; white halos are produced mainly by reflection. (see below left)
With just a Laser Pointer and a Laser Printer each of your students can now generate their own double-slit patterns — and it’s FREE!  Click here
Produced by irregularly-sized droplets, these coronal fragments appear as wisps of iridescent pastel colors in clouds.

Cool Coronae:
To produce a corona, simply breathe on a cool piece of glass. More often than not, a corona will be seen by looking at a light source through the water droplets that condense on the glass. If you wear eyeglasses, simply exhale on one of the lenses. When you look at a light source through the lens you will see a corona whose colors change with time. Since the colors produced depend on droplet size, the colors change as the droplets get smaller and finally disappear.

You may not even have to breathe on glass to observe coronae. You may see them through a fogged windshield or on steamed up glass in the bathroom.

Iridescent Cloud in a Bottle:
Iridescent coronae are often produced by the water droplets that make up thin clouds. So to produce coronae it would seem that all you need to do is make a cloud. Using a gallon jar, a rubber glove, some water and a match, you can do just that. First cover the bottom of the jar with a thin layer of water. Drop a lit match into the jar. Quickly place the fingers of the glove inside the jar and stretch the open end of the glove over the mouth of the jar. Put your fingers the glove and pull the glove outside the jar. Presto! You should see a wispy cloud inside the jar.

To observe a corona, shine light from a bright source such as a slide projector or flashlight through the jar. Initially smoke particles will scatter all wavelengths of light producing a white cloud. As the smoke disappears, leaving smaller droplets, pastel colors will be seen at certain viewing angles. You’ve just observed your first corona in a bottle!

The figure above shows a rare atmospheric optical phenomenon known as a circumhorizontal arc. Caused by the refraction of light through the ice crystals in cirrus clouds, it occurs only when the sun is high in the sky, at least 58° above the horizon.

Reminiscent of a rainbow, the circumhorizontal arc is produced only when the ice crystals making up cirrus clouds are shaped like thick plates. Furthermore, these plates must have their faces parallel to the ground. The chances of having all these conditions satisfied are low, hence the infrequent observation of this amazing optical phenomenon.

### Other Cool Sky Stuff

In the photo shown here, the Aurora Australis is seen over the National Science Foundation’s Amundsen-Scott Pole Station.

Aurora Borealis

The Northern and Southern Lights, or more formally Aurora Borealis and Aurora Australis respectively, are produced when charged particles from the Sun pass through the Earth’s upper atmosphere. The high-speed particles energize gas molecules which in turn emit the ephemeral colored lights we associate with the Aurora.

This image is courtesy of UK photographer Rich Lacey. While spending time in Northern Canada Rich had to opportunity to capture the best Aurora photos we’ve seen. You can see more of his images and order prints for your class on his web site at www.richlacey.com

### Light Pillars

Often seen in very cold weather, light pillars seem to be beaming up from terrestrial light sources such as street lamps. Many initially mistake light pillars to be searchlights. Light pillars result from the reflection of light from hexagonally-shaped, plate-like crystals. These crystals fall with their flat surfaces in a horizontal orientation. The flat surfaces serve as mirrors, reflecting the sun’s light downward.

A sun pillar is a vertical shaft of light extending upward or downward from the sun. Like light pillars, they are produced when sunlight reflects off the surfaces of plate-like ice crystals. Sun pillars are usually seen at sunrise or sunset when the sun is low on the horizon.

Northern Lights! http://www.northern-lights.no/

Atmospheric Optics http://www.atoptics.co.uk/

Weather Optics http://www.aws.com/aws_2001/schools/wx_mania/063003.htm

## Haunted Laboratory: Halloween Physics Part II

Since publishing the last Halloween edition of CoolStuff, we have received many positive comments from teachers who have created their own haunted laboratories. We have learned that teachers not only share their displays with their own ghosts and goblins but often open up their labs to the entire school or community. Therefore, we’ve decided it’s time to revisit the Haunted Laboratory in search of even more science tricks and treats.

This edition of CoolStuff features some new exhibits designed to put a spooky spin on science. The displays, selected with ease of construction in mind, use materials, and apparatus found in most science storerooms. Specialty items, such as a green laser pointer, may be readily obtained from Arbor Scientific. To make the most of your Haunted Laboratory experience, we’re offering the following suggestions from physicist and Haunted Laboratory pioneer Thomas Zepf.

• The lab should be as dark as possible. Quite often the exhibits provide sufficient ambient lighting. However, safety is always the first concern. If necessary, use additional lighting to ensure that students are able to safely navigate the room.
• Many pieces of standard laboratory apparatus can be modified for use in a haunted lab. Just let your imagination run wild!
• To set the mood, decorate the room with a variety of Halloween items. Artificial spider webs, plastic bats, pumpkins, and masks are always good.
• The overall experience is enhanced by playing Halloween music or recordings of scary sounds. Such recordings are inexpensive and readily available. You may find that your library has a wealth of suitable recordings.
• Consider using students as facilitators. Enthusiastic and knowledgeable student helpers make the experience more enjoyable and meaningful for visitors. As is always the case, the best way to learn something is to teach it. In addition to serving as docents, students will derive a great deal by assisting in the setup of the laboratory.
• Consider inviting students from other schools and members of the community to your haunted lab. We are certain you will find the experience gratifying.

(Note: To read Professor Zepf’s account of his experiences with haunted laboratories, see the October 2004 edition of The Physics Teacher magazine.)

Haunted Lab Exhibits

We have all seen a reflection of ourselves in a kitchen or bedroom window at night. Because we are in a room that is well illuminated and the outside is dark, light reflected by the window predominates over the light coming through the window from outdoors. We think of the glass as an ordinary mirror, often forgetting that the people on the outside can see us very clearly. This is essentially the “one-way mirror” effect used in situations where an observer wishes to remain hidden. In police stations and department stores, partially silvered panes of glass behave as mirrors to those on one side. To those in a dark room on the other side, the glass functions as a window.

The device shown above demonstrates that a transparent material may be an effective reflector as well as transmitter of light. It also illustrates that the object and the image produced by a plane mirror are equidistant from the mirror’s surface. These two phenomena make the morphing of two faces possible by simply adjusting the intensity of the light falling on two individuals sitting on opposite sides of the glass. The results of this simple exercise can be very freaky, to say the least! Things can become even spookier if one of the participants wears a Halloween mask.

Image courtesy of Pati Sievert at Northern Illinois University

The apparatus consists of a sheet of Plexiglas and two floodlights controlled by dimmer switches. Users sit on opposite sides of the Plexiglas and adjust each light’s intensity until the image of one person’s face merges with the image of their partner’s face. It should be noted that only one participant at a time is able to observe the effect.

The photos above represent two possible setups: a more polished version of the device (Top image) and a very simple setup (Bottom image). In the simpler device, the glass from an old ripple tank serves as the reflecting surface. Two dimmers (Variacs also work well) are used to control the brightness of two standard light bulbs or two flood lamps. An eerie effect may be achieved by using different colored bulbs (e.g. orange and blue) in each of the sockets.
Instructions:

1. Lights on apparatus may have to be adjusted so they shine on the students who will be sitting on each side.
2. Have two students sit, one on each side of apparatus, approximately the same distance away from the apparatus.
3. Students need to line up their eyes and nose as best as they can.
4. Have students adjust the lights so that one is relatively bright while the other is dim. Have observers view the results from the bright side. Remind the student sitting on the dim side that they will not observe the effect.
5. Have students reverse the lighting conditions so the student initially on the dim side will have an opportunity to see the unusual effect.

### Monsters of the Deep in a Drop of Pond Water

A drop of pond water at the end of a syringe or eyedropper can be treated as a small spherical lens. The light beam that falls on the drop refracts as it passes through the water-air interfaces at both sides of the droplet. The shadows of the small creatures contained in the suspended drop of pond water are magnified up to 1000 times by the liquid lens.

The top image shows the setup for the Monsters of the Deep using a ring stand to hold and adjust the drip rate of the eyedropper.

The bottom image shows the darkroom results of the various creatures we found in our “Swamp Water”.

Image courtesy of the 2006 Arbor Scientific Staff Science Olympics. Our drop of pond water was full of creatures. It was like a Monster convention!  Because of extreme lighting conditions, it was difficult to do it justice.

This activity was inspired by an article in The Physics Teacher magazine by Gorazd Planinsic.

Water-drop projector Physics Teacher 39, 76 (2001)

Watch the laser light on the screen. You should see the magnified shadow images of microscopic organisms floating and moving within the projected light. Small single-cell animals like paramecium appear as dark spots surrounded with interference fringe contours. Larger animals such as mosquito larvae, Cyclops, or water fleas appear like real monsters on the screen.

### Pepper’s Ghost

The one-way mirror (see “Freaky Faces”) has long been used in the performing arts, magic acts, and amusement parks. It was exploited for theatrical purposes in the mid-nineteenth century when J.H. Pepper produced what appeared to be a hovering apparition on a London stage. By angling a large glass plate between the stage and the audience, Pepper produced an image of an actor who seemed to hover over the stage (see figure below). To create the illusion, a well-illuminated actor was located in front of and below the stage, out of the audience’s view. A light source of variable intensity was employed to make “Pepper’s Ghost” come and go at will. A darkened theater insured that unwanted reflections would not reveal the presence of the glass. Similar techniques are currently used in amusement parks and museums to startle and amaze unsuspecting visitors.

You can create a simple version of the Pepper’s Ghost illusion with a sheet of glass or Plexiglas, a candle, a drinking glass and a cardboard box. First, remove the flaps from the mouth of the box. Spray the inside of the box with black paint or line it with black construction paper. Place the sheet of glass or plastic over the mouth of the box. Ideally, the sheet of Plexiglas should be the same size as the opening; however, this is not necessary. Place the box on its side with the mouth of the box facing you. Fill the glass with water and place it inside the box. Now cover the opening with the Plexiglas sheet. You may wish to tape it in position. Finally, place a lighted candle in front of the box so that the candle is in front of the drinking glass and both objects are equidistant from the Plexiglas. Now turn off the room lights. When you light the candle, the flame will appear to be burning in the water! The photos below are physicist Dave Wall’s Pepper’s Ghost device.

A setup resembling the original Pepper’s Ghost apparatus may be constructed from a large sheet of glass or Plexiglas, a light source, and a person in costume or a scary mannequin. The reflecting sheet should be slightly taller than the observer. A smaller sheet may be used by raising it with supports.

A glass of water is placed at the exact location of the reflected image.

Here the actual candle is shielded from the viewer by a partial cylinder.

A costumed person or mannequin stands in front of the Plexiglas. When the person or mannequin is illuminated, a ghostly image will appear behind the clear reflecting surface. It’s fun to watch a friend go behind the Plexiglas and put their arm around the image or pass their hand through the body of the image.

Key Concept:
This exhibit consists of a reverse mask. That is a mask that is intentionally set up to be viewed from the wrong side. The display demonstrates that our eye-brain system is conditioned to interpret all faces as convex even when they are not.

Teacher Instructions:
The centerpiece of this display is a white translucent mask. In the photo, a mask of Einstein is shown (see below for details), but any inexpensive mask will work.  Simply place a standard light bulb behind the reverse mask, stand approximately 10 feet away and view the mask with one eye closed. Presto! The concave face will appear convex! Once the illusion is established, open both eyes. If you are like most people, you will still see a convex face. Now move from side to side. The face will follow you wherever you go!

When we look at a reverse mask, our eye-brain system perceives a normal face for two reasons: shadowing and expectation. We are accustomed to seeing subtle shadowing when human faces are illuminated from above. These shadows tell us that the face is in relief. Similar shadowing is produced when light passes through the translucent mask. Perhaps an even more important factor in establishing a sense of convexity is expectation. Since we virtually never encounter a concave face, our previous knowledge tends to override reality. That is, we tend to see what we believe rather than believe what we see.

As we move past the mask, we see more of one side of the mask than the other. Based on experience, we interpret this change in perspective as being caused by the rotation of the face rather than a change in viewing angle.

If we get too close to the mask, we receive visual information that destroys the illusion of convexity. For example, reflections from the plastic surface often reveal the true curvature of the mask.

Image courtesy of Pati Sievert at Northern Illinois University

## Einstein Alive

In Stock SKU: P2-6000
\$20.00

Student Instructions:
Stand a few feet away from the mask. With one eye closed, look at the mask. Does the mask appear concave or convex? Now open both eyes. Does the mask continue to look convex? Finally, with both eyes open, move from side to side. Can you escape the gaze of the mask?

### Ghostly Messages

Using a little Tide or Cheer and a black light, you can make messages and scary images appear seemingly out of nowhere. Many liquid detergents contain fluorescent dyes. These dyes serve as whitening agents that are intended to make yellowed clothing look white and brighter. When illuminated with ultraviolet light, these liquids fluoresce, giving off a cool blue light. By mixing roughly equal parts of detergent and water, you can produce a “paint” that is invisible in white light but fluoresces dramatically under black (ultraviolet) light. You may write a message or draw with your finger tip or a brush on a tabletop, wall, or sheet of paper. Once it has dried, your painting will be virtually invisible in normal light but readily visible in ultraviolet light.

Image courtesy of Pati Sievert at Northern Illinois University.

### The Oozing Flesh Display

This exhibit illustrates an effect often called the “Waterfall” or “motion aftereffect” illusion. If you stare at a waterfall, cells in the eye that sense downward motion becomes fatigued. If you shift your eyes to the side of the waterfall, the nearby rocks appear to float upwards because the cells that sense upward motion are not tired and send a stronger signal to your brain. This causes your brain to conclude that the rocks are moving upwards. The same effect can be observed if you stare at a road while in a moving car and then divert your gaze to a stationary object.

Similarly, eye cells that detect rotation in one direction become fatigued and are overpowered by cells that detect rotation in the opposite direction causing the sensation of reverse movement.

To duplicate this effect, print out the dotted disk and attach it to a slow-moving turntable. An old record player set at 33/13 or 16/2/3 works well. You can get your skin to seem to ooze by following these steps: Stare at the center of the spinning disk for 30 seconds.
After 30 seconds of staring, look at the palm of your hand, face or arm.
Does your flesh seem to be oozing? Does it appear to turn in the same direction as the disk or in the opposite direction?

Try looking at objects in the room after staring at the disk for 30 seconds.

If you don’t have a turntable, mount the disk on a piece of circular cardboard. Then attach the center of the disk to a pencil’s eraser with a thumbtack. Spinning the disk at a fairly constant rate by hand is guaranteed to get your skin crawling!

### See more Haunted Physics Lab Exhibits

We started discussing the idea of the Haunted Physics Lab in CoolStuff #11, released October 2003. You can see other haunted exhibits from that issue and much more by visiting the CoolStuff Archives. Every issue is added to the Archives and is searchable by topic, title and volume number.

Did you read Haunted Laboratory: Halloween Physics part 1? if not check it out.

## Lab in a Bag: Take Home Science

Although a school’s science laboratory is the traditional arena for exploration and experimentation, other venues, such as interactive science centers, do exist. For some time now we have been taking advantage of yet another setting: the home. Using simple materials, our students are encouraged to do science experiments with family and friends. The benefits of at-home science activities are many. They increase the time students are thinking about and doing science. Since many of the explorations focus on counterintuitive phenomena, students delight in sharing unexpected outcomes with others. Needless to say, parents love seeing what their children are doing in school.

Quite often the materials needed to investigate physical phenomena at home may be found in the kitchen or workshop. When more specialized equipment is needed, we create a “Lab in a Bag” by packing required materials in a plastic food-storage bag. Using the “lab in the bag” approach, students take home simple materials relating to a given concept in Zip-Loc® bags. Everything needed to investigate phenomena ranging from electromagnetic radiation to Newton’s Laws is contained in a single plastic bag.

The “Lab in a Bag” experiments are intended to be engaging, thought provoking, and enjoyable. While fun is not the principle goal of science education, these activities allow students, and their families, to experience science in a less-structured, more playful manner. All activities are designed to be straightforward and materials are chosen with safety in mind. The low-cost nature of the simple equipment used in these kits eliminates worry about loss.

Prior to presenting the students with their first activity, we send a letter home to parents explaining the purpose and nature of the activities. The letter also informs parents that their child will receive credit upon the return of a signed sheet indicating the parents’ or guardians’ involvement in the activity.

The take home labs may be divided into two categories. The “Lite Science” labs involve short investigations that may be carried out with additional materials found in the home. “Lab in a Bag” experiments require the use of materials packaged by teachers. The following are examples of both types of explorations. We hope you enjoy sharing these activities with your students and their families.

Don’t try this at home!

Is it just me, or does the phrase “where there’s a will, there’s a way” go just a little too far here? (Download and watch the video of Bottle Rocket Man)

### “Lite Science” Experiments

The Color Mixing Turbine – Overlapping Color in Time

Due to a phenomenon known as persistence of vision, our retina retains an image for a short time after the source of light has come and gone. Using persistence of vision, it is possible to combine colors by presenting them to the eye in rapid succession. If for example, a flash of red light impinges on the retina, the sensitive cones that are activated by the light continue sending signals to the brain for a fraction of a second. If a source of green light strikes the retina within this time, the brain will perceive yellow, the additive combination of red and green.

The color mixing turbine provides a simple yet elegant way of demonstrating the use of persistence of vision to achieve additive color mixing.

The following steps will guide you through the construction and use of the turbine.

Bend two corners of one of the black cardboard squares as shown in the figure above.

Attach a green sticker to one side of the card and a red sticker to the
other. Make certain that the two stickers have overlapping areas.

Gently hold the card by corners B and D using two fingers. By blowing on
the concave blade, the cardboard can be made to spin. The alternating
colors act as flashing red and green lights, the combination of which
produces the sensation of yellow.

Experiment with the other stickers provided. Record your results below.

Red sticker/Green sticker yields ____________________

Red sticker/Blue sticker yields ______________________

Blue sticker/Yellow sticker yields____________________

This activity was inspired by an article in The Physics Teacher magazine by Adolf Cortel titled Simple Experiments on Perception of Color Using Cardboard Turbines, Physics Teacher 42, 377 (2004).

### Friction Balance

Balance a meter stick or other long rod on your forefingers so that a finger is on each side of the center of the rod. The exact position of each finger is not important. In fact, you don’t want them to be the same distance from the center of the rod. Now slide your fingers toward each other. Watch your fingers carefully as they move inward. Where do your fingers meet? Try it again, this time with your fingers in different starting positions. Where do your fingers meet now? Can you explain your observations?

You may wish to try this experiment with a variety of objects, e.g., a broom, baseball bat, etc.

### Mind Over Matter Pendulums

Resonance occurs when the frequency of the driving force acting on an object equals the object’s natural frequency of vibration. The shattering of glass with sound shown in an old Memorex® tape commercial and the Tacoma Narrows Bridge collapse are perhaps the most frequently cited examples of resonance. In each case, the object’s amplitude of vibration increased dramatically through the efficient transfer of energy.

The power of resonance can be summoned in a less destructive, but equally impressive demonstration. The “mind over matter pendulums” will leave observers in awe as you set each of three pendulums swinging at will, apparently through the use of your psychokinetic powers.

The pendulums are constructed from paper clips suspended from three strings of different lengths. The lengths of the strings are not important. One end of each string is tied to a paper clip, the other end to a drinking straw. The demonstrator holds one end of the straw and asks unsuspecting observers to identify the pendulum they want you to set into motion using only brain waves. Once the pendulum is selected and you have requested complete silence from the audience, begin chanting “come on, come on.” To prevent accusations of breathing on the pendulum, hold your free hand in front of your mouth.To set the designated pendulum swinging, you must imperceptivity move the end of the straw at the pendulum’s resonant frequency. This is easier to do than it sounds. A few motions of the straw will reveal if the chosen pendulum is responding. If not, adjustment is instinctive. When resonance is obtained, energy will be transferred to one pendulum efficiently, but not the other two. If your hand motions remain undetected by the audience, you will have everyone baffled until you finally decide to share the science behind the scam.

### Great Balancing Act: To be taken with a grain of salt

An object in equilibrium may either be stationary (static equilibrium) or moving with a constant velocity (dynamic equilibrium). The equilibrium condition requires that there be no unbalanced forces or unbalanced torques acting on an object.

One of the most striking demonstrations of static equilibrium may be performed with only a beaker or drinking glass and a little salt. First, make a little mound of salt, roughly 2 cm wide and 1 cm tall, on a table top. Next grind the edge of the bottom of a beaker into the salt until the beaker remains stationary and poised at angle. The beaker is now in static equilibrium. Slowly and gently blow on the salt until only a few grains remain between the beaker and the table top. When this occurs, describe the forces and torques acting on the beaker. Dissolve the remaining grains of salt by pouring water on the table near the beaker’s point of contact with the table. What happens at the instant the water reaches the grains of salt? Why does this occur?

Repeat the experiment by increasing the size of the object. You might want to try larger vessels. 1000 ml beakers are always a challenge. The largest known object used in this demonstration is a trash can.

### Silver Egg Demo

Use tongs to hold a normal egg in a candle flame until it is covered with soot. Drop the soot-covered egg into a glass of water. A considerable amount of the light traveling through the water is totally internally reflected when it encounters an air layer that adheres to the soot. Since most of the light is reflected, the egg appears to have a silvery, shiny surface. The egg will appear silvery until the air dissolves into the water, which only takes a couple of minutes. Look closely to observe what happens to the small fraction of light that passes through the air layer.

Permanent Thin Film Colors Light incident on a thin film, such as a thin layer of gasoline on water, will be reflected from the top and bottom surfaces of the film. When the reflected light waves exit the film, they interfere. This interference gives rise to the iridescent colors often associated with soap bubbles and oil slicks. These colors are as short-lived as the films that produce them. However, there is a way to capture the beauty of a thin film for posterity. Using a drop of inexpensive clear finger nail polish and a sheet of black construction paper, a thin film and its attendant colors may be made permanent.

Obtain a pan or glass dish large enough to accept a 4” X 4” sheet of black construction paper. After filling the container with water, put the construction paper in the water making certain that it is completely submerged. Use the nail polish applicator brush to apply one drop of nail polish to the center of the water. The nail polish should quickly spread out over the surface of the water. After the nail polish has stopped spreading, slowly lift the construction paper out of the water. The nail polish should adhere to the surface of the paper. Allow the paper to dry. What do you observe on the surface of the dried paper? Remembering that the nail polish was colorless, can you explain the origin of these colors? Did you note that the colors change as you tilted the paper? Why does this happen?

### Construct a Kaleidoscope

This activity is a hands-down favorite of our students! This “Lab in a Bag” includes three 1”X 6” mirrors and an instruction sheet. The sheet describes kaleidoscope construction and offers suggestions for creating a variety of objects to be viewed through the scope. The sheet also provides a brief explanation of image formation by the kaleidoscope and a history of the device. The resulting kaleidoscopes are absolutely stunning! Students often give their finished kaleidoscopes to family members and friends as gifts.

The mirrors used are cut from standard mirror tile available at any hardware or home supply store. You may cut the mirrors at home using a glass cutting tool; however, many hardware stores will cut the glass for free when they learn of your mission.

The simplest kaleidoscope is constructed by simply taping the three mirrors together with masking or electrical tape. The mirrors are placed face down and the tape is applied over small gaps left between the mirrors. These spaces allow the mirror assembly to be folded into a triangular shape. Without the gaps, the mirrors will bind.

When no object is permanently attached to the far end of the three-mirror system, the device is called a teleidoscope. View your world through the teleidoscope and be amazed! Everything seen through the teleidoscope is transformed into a beautiful, multi-faceted pattern. Attaching an object such as a decorated ping pong ball or test tube containing water and colored beads to the end of mirror system formally turns your teleidoscope into a kaleidoscope.

### Exploring Color

This “lab in a bag” allows students to explore principles of additive and subtractive color mixing. Along the way, they are made aware of examples of color mixing going on all around them. Each student is given six color filters (red, green, blue, cyan, yellow and magenta) and a pair of inexpensive diffraction glasses. The color filters need not be large. 2” X 2” squares will suffice. A small piece of diffraction grating taped over a hole punched in a file card may be used in lieu of diffraction glasses (see figure).

INFRARED GOGGLES FOR UNDER \$10: A Human IR Vision Experiment by Bill Beaty http://www.amasci.com/amateur/irgoggl.html

Seeing Infrared and Ultraviolet by Paul Doherty http://www.exo.net/~pauld/activities/astronomy/seeingir.html

## Haunted Laboratory: Halloween Physics

Each October the physics teachers at New Trier High School in Winnetka, Illinois treat their students and the public to a haunted science laboratory. Visitors learn science while having fun as they are confronted with a maze of displays that demonstrate optical, acoustical, mechanical, electrical, and perceptual phenomena in the context of Halloween. Scientists and non-scientists alike become engaged in trying to understand the science behind the fun.

As strange as it may seem, science and Halloween do have something in common: they both exemplify our innate fascination with the mysterious. The concept of a haunted science lab was conceived of over twenty years ago by Creighton University physics professor Tom Zepf. It began as a collection of activities in Light, Color and Lasers, a core-curriculum course he was teaching at Creighton. Gradually the annual Halloween event evolved into laboratory-based experience consisting of over three dozen displays demonstrating a wide range of physical principles, all of which invite interaction.

In the last few years, the concept of integrating science with fright has spread to other educational institutions (e.g., New Trier High School and Deerfield High School, both in Illinois, and Luther College in Iowa). In the process, displays illustrating perceptual concepts have become a part of the annual exhibitions. Like light and color, visual perception lends itself extremely well to the Halloween theme. The inclusion of psychological and physiological phenomena has made the haunted lab relevant to students of psychology and biology while demonstrating connections between the sciences.

We now offer some examples of favorite haunted lab exhibits. The beauty of these displays is that they are based on apparatus found in most science storerooms. With slight modification, many devices commonly used in the traditional science laboratory may be transformed into something spooky.

Student Activities

### 1. Mirrorly a Ghost

Key Concept

An object placed at the center of curvature of a concave mirror will produce an inverted real image whose size and distance from the mirror exactly match that of the object.

Teacher Instructions

Based on the standard spherical mirror demonstration, a large concave mirror is used to produce a ghostly apparition in an exhibit we like to call “Mirrorly a Ghost.” An illuminated, inverted plastic ghost is mounted at the center of curvature of a large concave mirror. The mirror forms an erect image of the ghost directly above the object ghost. The illusion is made complete with the addition of arms, shirt cuffs, and a white sheet extending from the image to the floor.

## Concave Mega Mirror

In Stock SKU: P2-7150-02
\$159.00

Image courtesy of New Trier

High School’s Haunted Lab

Student Instructions
Look at the ghost before you. Is it real or is it virtual? Hint: The actual ghost is located just below the ghostly image you see. Hint #2: A concave mirror is used to produce this spooky illusion.

### 2. Apple Oscillators

Key Concept

When two pendulums have the same length, they will oscillate at the same frequency. Scientists say that the two pendulums exhibit resonance. If the pendulums are coupled in some way, say by a string, the gentle tugging produced by the connecting string will efficiently transfer energy back and forth between the two pendulums.

Teacher Instructions

Two apples suspended from strings and linked by a string or rubber band can be made to stop on command. Knowing that the apples’ energy is transferred back and forth allows the operator to predict when each apple will stop.

Student Instructions
Start one apple swinging by pulling it back a few centimeters. Now carefully watch what happens. Notice that the amplitude of swing of the apple you released is decreasing while the apple initially at rest is starting to move. Will the first apple ever stop swinging? You’ll have to watch and see!

### 3. Pumpkin Pendulum

Key Concept

Conservation of Energy: Energy cannot be created or destroyed, only changed from one form to another.

Teacher’s Instructions

A pendulum with a pumpkin as a bob is used to demonstrate the conservation of energy. The pumpkin is supported from the ceiling with a rope. A volunteer stands with back against the wall. The pumpkin is brought back to the volunteer’s nose and then released. The trick is to remain motionless with your eyes open as the pumpkin first swings away and then returns.

Special Note: Be sure that you make a notice to the pumpkin user not to give it an initial push! This could have a big impact on the participant!

Student Instructions

With your back against the wall, hold the pumpkin so that it just touches the tip of your nose. Now with your eyes wide open, release the pumpkin and wait for it to return. Try not to flinch as the pumpkin comes hurling back at you!

### 4. Ghostly Apparition

Key Concept

Real images are formed where reflected light rays converge.

Teacher’s Instructions

At the heart of this demonstration is the Mirage. The device consists of two inward facing concave mirrors with identical focal lengths. The two mirrors fit together forming a shape reminiscent of a flying saucer. The ghost, placed on the surface of the lower mirror, produces a real image in the plane of a hole cut in the upper mirror.

Student InstructionsGhosts are untouchable and this exhibits proves it! A tiny ghost figurine is seen perched on top of what appears to be two bowls inverted on each other. When you attempt to touch the ghost, your fingers go right through it!

### 5. Spooky Anamorphic Art

Key Concept

Anamorphic images are purposely distorted and require a cylindrical mirror to make them intelligible.

Teacher Instructions

Anamorphic images of ghosts, goblins, and other things that go bump in the night may be produced using computer graphics programs. For example, Print Artist from Sierra allows you to wrap images into the required semicircular shape. This program also has a library containing many Halloween images.

We encourage you to have your students produce their own Anamorphic Halloween images using the grids found at our link. Instruct them to first draw a picture on the rectangular grid. Then tell them to transfer their drawing, point by point, onto the cylindrical grid. As they do so, their image will become distorted. However, it will appear normal when viewed with a cylindrical reflector.

Student Instructions
Place a cylindrical reflector at the center of the distorted image. Look at the image of the distorted image in the cylindrical mirror. What do you see? Does the image now appear normal?

Key Concept

The eye-brain system is conditioned to interpret all faces as convex even when they are not. Since we virtually never encounter a concave face, we tend to see what we believe rather than believe what we see.

Teacher Instructions

This reverse mask illusion relies on our expectations. We tend to see what we have learned to see, in this case a convex face. You may wish to have your students experiment with lighting. Suggest that they try illuminating the mask from both top and bottom and from behind if the mask you are using is translucent. We are accustomed to seeing the subtle shadows produced when convex faces are illuminated from above. The same shadowing results when light from below shines on a concave face. Light passing through the mask will produce a similar precept.

Student Instructions
Stand a few feet away from the mask. With one eye closed, look at the mask. Does the mask appear concave or convex? Now open both eyes. Does the mask continue to look convex? Finally, with both eyes open, move from side to side. Can you escape the gaze of the mask?

### 7. Ectoplasm

Key Concept

Gas molecules that fill the globe are stripped of their electrons by electromagnet waves emitted by a transmitter at the bottom of the globe. An eerie discharge is produced when the electrons recombine with the ionized gas molecules.

Teacher Instructions

You may wish to bring a fluorescent lamp near the globe. The electromagnetic radiation responsible for ionizing the gas molecules with the globe will also excite the phosphors that line a fluorescent lamp. Bring a radio near the globe. You should be able to hear the noise produced by the EM waves emanating from the globe.

Student Instructions
Change the electrical discharge pattern by gently touching the glass with your fingers. Placing your hand on the sphere may reveal the lines in your palm.

### 8. Magic Wand

Key Concept

An image is visible when focused light is reflecting off of something. Also, the brain retains images for a fraction of a second in an effect known as Persistence of Vision.

Teacher Instructions

Focus a 35 mm slide of your choice (I like to use a ghost image) on a sheet of white paper hung in the middle of the room. After obtaining a sharp image, remove the paper. Use a tape line on the floor to indicate the position of the paper. Students will rapidly swing a long dowel rod up and down in the area previously occupied by the paper. The dowel will reflect the focused ghost image one bit at a time, and the fast swinging will cause the whole image to persist in the observer’s brain.

Student Instructions
Hold the “Magic Wand” in your hand and wave it rapidly up and down over the tape line on the floor. Look at the side of the wand that faces the projector. What do you see? Wave the wand faster and slower. When can you see the whole image at once? What happens to the image when you stop waving the wand?

### 9. Ghost Brains

Key Concept

We can see a transparent object when its index of refraction is different than the surrounding material. Ghost Crystals, also called Phantom Crystals, have an index of refraction almost exactly equal to that of plain water.

Teacher Instructions

Combine a tablespoon of Phantom Crystals and 2 liters of water in a large beaker or bowl. Let them soak overnight. The result should look like plain water.

Note: You will need 1 container of phantom Crystals.

Student Instructions
What do you see? Put your hand into the container of Ghost Brains. What do you feel?

This skeleton rotates only in one direction. However stare at it long enough and what do you see?

### 10. The Indecisive Skeleton

Key Concept

Two-dimension shadows of three-dimensional rotating objects do not contain enough information for the eye-brain to determine the object’s direction of rotation. Consequently, at any instant, the direction of rotation of the object’s shadow may change!

Teacher Instructions

This exhibit is relatively easy to set up and well worth the effort. A toy skeleton, or any other object for that matter, is attached to the shaft of a slow turning motor. Using a 35 mm slide projector, the shadow of the rotating skeleton is projected on a translucent screen. A blank artists canvas works beautifully.

Student Instructions
Stare at the shadow of the rotating skeleton. You will notice that the skeleton appears to be rotating in one direction, and then, all of a sudden, will appear to rotating in the opposite direction. However, it’s all in your head. The plastic skeleton casting the shadow always rotates in the same direction.