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.
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.
Making Your Own Rainbow I:
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.
Making Your Own Rainbow II:
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?
Making Your Own Rainbow III:
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.
Discussions on rainbows and the optics of the sky always lead to the topic of the electromagnetic spectrum.
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.
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.
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.
Note that a cardboard mask blocks the light not passing through the beaker.
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.
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.
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.
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.