This experiment works best in a room that can easily be made dark. Fill a shallow bowl or pan with water. Place a small mirror in the water with the reflective side leaning upward at an angle to the water’s surface. Turn the lights off and shine a strong flashlight through the water and onto the mirror. Prismatic colors will be reflected onto the wall or ceiling due to the refraction and dispersion of the flashlight beam. This is a simple way to demonstrate the difference between refraction, reflection and dispersion, which all students should understand before undertaking further experiments.
You may demonstrate how rainbows form by using a round, water-filled flask as a model of a raindrop. In the center of a piece of white cardboard, cut a hole roughly equal in size to the spherical part of the flask. Shine a projector lamp or a strong flashlight through this hole and into the flask. When the beam of light enters the flask it will refract, reflect off the back of the flask and then refract once again as it travels back through the water toward the light source. The resulting dispersion will cast a circular rainbow on the side of the cardboard facing the flask. On a wet day, countless water drops disperse sunlight in the same way, forming a circular arc whose lower half is hidden below Earth’s horizon. From a higher vantage point — an airplane, for example — it’s possible to see a fully circular rainbow.
On a clear day when the sun is low in the sky, have students spray a hose into the air while turning in a full circle. This will quickly demonstrate that an observer can see a rainbow only when the sun is behind him and the shadow of his head is visible in front of him. This shadow marks the antisolar point, or the spot that is directly opposite the sun. Rainbows form in a circular arc 40 to 42 degrees from the central antisolar point; the water droplets that produce the rainbow lie along the surface of a cone whose base is the rainbow’s arc and whose apex is the observer’s eye. One interesting consequence of this fact is that two observers standing side by side are each seeing a rainbow formed by a different collection of water droplets. This means that no two observers can ever see the same rainbow, except in a photograph. Of course, this also means that if you take a step to the left or right, you will see a different rainbow than you saw a moment before.
Fill a clear plastic rectangular box with water and use it as a prism to cast a rainbow on a sheet of white construction paper. Have students experiment with different angles, light sources and surfaces. For each experiment, ask them to write down or draw the colors in the order they appear. They should discover that the colors always appear in the following sequence: red, orange, yellow, green, blue, indigo and violet. Water refracts each wavelength of visible light at a slightly different angle: Red, which has the longest wavelength, refracts at 42 degrees. Violet, which has the shortest wavelength, refracts at 40 degrees. As an additional activity, have each student invent her own mnemonic phrase for the spectrum of visible light, such as “Racing On Your Grandmother’s Bicycle Is Valiant.”