As we enter the 21st century, perhaps no area of science touches our lives and the lives of our students more directly than optics. We speak on the telephone without realizing that our voices have been converted into digitally-encoded flashes of light that travel over miles of thin optical fibers. Information, whether it be music, images or text, stored digitally on CDs and DVDs is recorded and retrieved optically. At concerts and sporting events, giant screens consisting of thousands of light emitting diodes provide ultra-bright, high-resolution images of figures too small to be seen on the stage or field. State of the art infrared cameras send eerie nighttime images of battles thousands of miles away to our living rooms almost instantaneously.
Our understanding of outer space as well as much of the micro-world has been gained through optical exploration. The production of the color images that appear on television, computer, and movie screens relies heavily on optical and perceptual principles. And perhaps most importantly, over 80% of the information we receive from our environment is obtained through the most marvelous optical instrument of all, the eye. The study of light and color has always been the highlight of the year for my students. Optical effects are immediate, captivating, and as we have seen, incredibly relevant to our students' daily lives. Allowing students to explore with light, and our perception of it, produces a level of engagement that has to be seen to be believed!
This collection of laboratory experiences will take your students on a journey through the world of light, color and perception. The learning cycle approach will be employed to allow your students direct experience with optical phenomena and visual perceptual mechanisms. By way of hands-on experiments, your students will learn how light may be beamed, blocked, bounced, bent and even "stored." They will find out how soot can be transformed into silver and why the myriad colors on your television screen are really just in your head. Through some visual foolery, they will experience the perceptual paradoxes that occur when the brain is unable to make sense of sensation. And in the end, find that they have learned a new way of "seeing the light."
Optical Phenomena Smorgasbord
Station 1: The Shadow Screen
Key Concept: The slow emission of stored energy in the form of visible light is called phosphorescence. "Glow in the Dark" phosphorescent materials with long decay times are used to cause watch dials, safety markers, and many children's toys to glow long after the lights go out.
In a darkened room, provide students with any camera flash and a sheet of Phosphor Glow Paper. If a small fan is available, students may "freeze" the motion of the fan blades by placing the fan between the Glow Paper and the camera flash.
Turn on the flash unit. Place your hand or other object (keys, pen, etc…) on the shadow screen. Direct the flash toward the shadow screen and activate the flash. Remove the object from the screen.
What do you see on the screen?
How do explain this image?
What happens to the image after a period of time?
Why does this happen?
Have you seen this material anywhere else?
Images courtesy of New Trier Connections Project, New Trier High School, Winnetka, Illinois.
Station 2: Concave/Convex Reflector
Key Concept: Curved reflecting surfaces are capable of producing both real and virtual images.
Hang a colored ball from the Virtual Reality Mirror so that the ball hangs in the center of the mirror.
Pull the colored ball attached to the silvered hemisphere towards you and release it. Watch the images produced by the concave reflecting surface. Describe the changes in the image as the ball swings back and forth and side to side.
Stop the ball and allow it to hang straight down. Where is the image now?
How does the size of the image of the ball compare to the size of the ball?
Look at your reflection in the convex (back) side of the hemisphere. How do you look?
Now hold your finger or other object in front of the convex surface. Describe the image formed. For example, is it right side up or upside down? Is it larger or smaller than the actual object?
Give some everyday uses for convex mirrors.
Station 3: Over the Rainbow
As white light passes from one material into another, the constituent wavelengths (colors) travel at different speeds, sometimes causing the white light to break up into a spectrum of colors.
Place a clear plastic (the clearer, the better) box of water on the overhead projector. Close or cover the top lens so that no light is projected forward out of the projector. Arrange the projector so that students can see the spectra that come out of the sides of the box.
Examine the array of colors produced by the water-filled plastic shoebox. Can you think of two things that this array of color has in common with an actual rainbow?
List the colors, starting from the inside, present in this rainbow.
How do suppose this rainbow is formed?
How is an actual rainbow formed?
Are colors originally present in white light or is the water the source of the spectral colors in both cases?
Station 4: Anamorphic Art
Key Concept: Anamorphic images are purposely distorted during their creation and require reflection in a cylindrical or conical mirror to make them intelligible.
Anamorphic images such as these can be found in The Magic Mirror: An Antique Optical Toy, by McLoughlin Brothers. The book is available from major bookstores and includes a piece of Mylar that can be rolled to make a cylindrical mirror. A tail pipe (about 6" long, shiny, straight, with a flanged end) from the plumbing department of the home improvement store, also makes a good cylindrical reflector. Some computer graphics programs will allow you to create your own anamorphic art by printing pictures in an arc.
Select one of the distorted works of art. Place a cylindrical reflector at the center of the distorted image.
Examine the reflection of the painting by looking at the side of the cylinder. Describe the image produced by the cylinder. Is the image distorted or does it now appear normal? Use the reflecting cylinder to look at other distorted paintings.
These distorted pictures are examples of an art form known as anamorphosis. A small group of artists began working with anamorphic art during the Renaissance. Anamorphic drawings appear strange and almost unrecognizable to the unaided eye, while a reflecting cylinder reveals an image of normal proportions. This technique is essentially the reverse of a fun-house mirror, which creates a distorted image of a normally proportioned person.
Use the grids to create your own anamorphic art. Draw a figure of your choice on the square grid, then transfer your drawing, point by point, to the cylindrical grid. Color your work if you wish. To transform your distorted figure into a recognizable object, just look at its reflection in the cylinder!
Download the anamorphic art grid sheet
Station 5: UV Beads (Electromagnetic Spectrum)
Key Concept: Dyes contained in these beads are sensitive to ultraviolet light. When exposed to UV light, they change color.
Provide UV Beads, a small UV tube (black light), and other sources of light, including sunlight.
Examine the beads and describe their color.
Now place a few beads near the "black light" tube. What happens to the beads when they are exposed to black light?
Can you suggest an explanation for this behavior?
What happens when the beads are now longer exposed to light from the tube?
Hold the beads near a TV screen or computer monitor. If possible, take the beads outdoors so that they are exposed to sunlight. Try other sources of light. Which of these sources of light cause the beads to change color? Why do you think this happens?
Suggest an experiment that you could perform with the beads to test the effectiveness of suntan lotions.
Station 6: Vanna
Key Concept: We not only tend to believe what we see, but we also see what we believe. We are not accustomed to viewing the human face upside down. Consequently we assume, based on past experience, that the inverted face will be normal.
Obtain two identical copies of a photograph. Carefully cut three rectangles around the person's eyes (including eyebrows) and mouth in one photograph. Glue the photo which is missing the eyes and mouth to the left side of your mounting board. Rotate each of the cut rectangles (containing the eyes and mouth) 180 degrees so that the mouth and eyes are upside down and glue them back onto the photo they were cut from. (This can also be done with a computer.) Glue the uncut photo to the right side of your mounting board. The mounting board can be attached to a slowly rotating motor or some other means for rotating the pictures manually.
Watch Vanna as she rotates. Why do you think she changes from gorgeous to gruesome?
Station 7: Bogus Barrier
Key Concept: Passing light waves through a polarizing filter results in light waves that vibrate in a single plane. Two polarizing filters with their polarizing axis crossed, that is, at right angles, will pass no light.
Cut a wide "window" in opposite sides of a shoebox. Cut a piece of polarizing film into four pieces, keeping track of the directions of polarization. Attach the film to the windows as shown, so that the two pieces in each window are polarized differently, but films directly across on the opposite window are polarized alike. Place a small ball in the box. Replace the lid. Provide another piece of polarizing film at the station for student investigation.
Examine the inside of the shoe box by looking through the tinted windows on either side. DO NOT OPEN THE BOX! Note that a wall divides the inside of the box into two regions. Tilt the box so that the ball rolls back and forth.
Does the ball pass through or bounce off the wall?
Can you explain this mysterious behavior?
If you are totally baffled, you may take the lid off the box. To discover why the "bogus barrier" exists, look through each of the windows with one of the square Polaroid filters provided at this station. You may find rotating the filter while looking through each window quite revealing!
Notice the direction of the polarized film panels.
Box drawing is shown without the lid on!
Image shows a plastic fork, protractor, and an empty cassette case under the polarizing film.
Station 8: Stressed Out
Key Concept: Placing certain transparent materials, such as plastic, between two polarizing filters produces colored patterns that are indicative of stress in the material.
Provide two large polarizing filters and several clear plastic objects, such as transparent plastic forks and cassette cases.
Place a plastic fork between two polarizing filters. Hold the filter/fork "sandwich" up to the light. What do you see?
How does rotating one of the filters change the appearance of the fork?
Study how stress affects the appearance of the fork by pinching the tines of the fork together as you view the fork between the filters. What changes do you observe?
Now observe other transparent plastic object between the filters.
Station 9: Einstein Alive
Key Concept: With the proper illumination, a concave face may appear convex. This is due in no small part to the expectation we have that human faces are always convex.
Place the Einstein Alive "mask" so that observers will see the concave (inside) side. Light the mask from behind with a light bulb or small desk lamp.
Stand about ten feet back from "Einstein Alive", close one eye, and look at the face. With one eye closed, slowly walk to the left, and then to the right, while you look at the face. Does the face seem to follow you no matter where you go?! Try moving up and down. Can you escape his gaze?
Station 10a: Additive Color Mixing
Key Concept: Three colors of light-generally red, green, and blue- known as the additive primaries can be mixed together to obtain any other desired color.
Cover three gooseneck or flood lamps with red, green, and blue color filters. Attach each lamp to an adjustable power supply. (Adjustable power cords are available at home improvement centers.) Train the beams from the three lamps on a white screen so that they overlap.
Adjust the intensity of lamps until their combined effects produce white light. Red, green, and blue are called the additive primary colors. Why?
Block one lamp at a time with a piece of paper and observe the resulting color on the screen. In your own words describe the color produced in each case.
red light + blue light = __________________________
red light + green light = _________________________
green light + blue light = ________________________
The name usually given to the combination of red and blue light is magenta. Red and green light produce yellow. Green and blue light result in what is known as cyan.
To experience the wide variety of hues possible by mixing the three primary colors, slowly adjust the intensity of the lamps. Specifically, try to create the colors listed in the chart below. In each case give a "primary recipe" that others may use to create a particular color.
Station 10b: Partitive Mixing
Key Concept: If small dots of color are placed very close to each other, the eye-brain system will mix these colors additively. This is the basis for color production on a TV screen and computer monitors.
Provide a way for students to place a small drop of water on a computer monitor or TV screen.
Place a drop of water on a television screen or computer monitor. Now look closely at the droplet. Careful examination will reveal an array of colored dots. Describe the color and arrangement of the dots. Can you see the individual dots without the aid of a magnifying glass? If so, how far from the screen must you be before you can no longer resolve adjacent dots?
The use of color dots to form images on television or computer screens is referred to as partitive mixing. Partitive mixing relies on the eye's inability to resolve closely spaced objects. In the case of a TV screen, the objects are red, green, and blue light-emitting phosphors. Phosphors for each of the primary colors are clustered together in groups of three. In all, there are over 200,000 of these primary clusters. At a sufficient distance from a TV screen, the eye interprets each triad of colored dots as a single color.
Which phosphors are emitting light when a television screen or computer monitor is white? Black?
Station 11a: Subtractive Color Mixing
Three colors of filters or pigments- often cyan, magenta, and yellow- can be absorption. This process is known as subtractive color mixing and is used in color printing and watercolor painting.
Provide color filters that students can stack and look through. Part b requires printed material with cyan, magenta, yellow, and black test squares on the side. Color printed boxes from the grocery store often have these squares under the bottom flap.
Hold a green and cyan filter together so that light from the lamp passes through both filters before entering the eye. Carefully observe the color(s) are visible through both filters. Record your observation below. Repeat this procedure with the combinations of colored filters listed below.
green + cyan = _______________
green + yellow = _______________
green + magenta =_______________
red + cyan =_______________
red + yellow =_______________
red + magenta =_______________
blue + cyan =_______________
blue + yellow =_______________
blue + magenta =_______________
red + green =_______________
red + blue = _______________
green + blue =_______________
Your observations may now be used to deduce the transmission characteristics of cyan, yellow and magenta filters. In terms of red, blue, and green, list the colors that are transmitted by each of the following filters:
cyan + yellow + magenta:______________________________
Cyan, yellow, and magenta are called the "subtractive primary colors." A cyan filter may be thought of as a "minus red" filter because it absorbs red light.
What color does a yellow filter absorb?
A magenta filter?
Station 11b: Color Printing
Both colored filters and pigments selectively absorb certain colors. Cyan, yellow,
and magenta are referred to as the subtractive primaries because in the proper combinations they may be used to produce any color in the spectrum. For this reason, cyan, yellow, and magenta are used in painting and color printing.
Examine the flaps on the boxes provided. There you will see cyan, yellow, and magenta "test dots" indicating the colors used to print the box. These same colors are clearly visible on the color inkjet printing cartridge. Use a magnifying glass to view a color picture in a magazine or book or a colored image produced by a color inkjet printer.
What do you observe?
Ok the smorgasbords done...what now?
Many times students miss some important aspect of an activity. Therefore, it is often good for the teacher to repeat some of the activities done by the students as class demonstrations. In this way the teacher can focus student attention on the essential element of selected activities. Drawing on students to explain what they see and why they think it happens gets them actively involved in a communal learning process while allowing the teacher to assess understanding.
In addition to revisiting smorgasbord activities, the teacher can further amplify basic principles through the use of novel demonstrations. These demonstrations may be used to further clarify concepts and illustrate real-world applications of the basic principles being studied.
Silver egg setup The "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.
In Station 11, students experimented with color subtraction. You can reinforce their observations with a more quantitative demonstration.
Color Subtraction Demo
Print four separated color CMYK images. Print these images as separate transparencies. Download Color Subtraction Demo PDF.
Place the magenta, cyan, yellow, and black images of the Palace on top of each other so that the four images are perfectly aligned. Now place the combination on the stage of an overhead projector or light table. What do you see?
How does this combination of images produce a full-color image? (Hint: think subtractive color mixing.)