Submodule 4: Rainbows
Steve Beeson, Arizona State University

Why do we see rainbows?

Everyone has seen a rainbow. They are arguably one of the most beautiful displays of nature, and they seem to come in many different sizes, situations, and settings. But what is a rainbow?

If you ask, most people will say that a rainbow is "light going through raindrops" or something to that effect. That is correct, but it's not the complete answer. In this lesson we will learn how and why rainbows form and how you can make one yourself in the classroom.

At home, put a glass filled with water near the edge of a table that is bathed in sunlight or some other parallel light source. As in the drawing below, a "rainbow", or spectrum, should appear on the floor near the glass.

In this case, the glass of water is acting like a prism and is dispersing the white light into its many colors.

What if we could also make the water act like a mirror?

Let's repeat our experiment with the water and the light.
This time, we'll use some materials from the Patterns lab. We'll need a powerful light source, an Erlenmeyer flask (a round-bottomed flask), water, and a large piece of white cardboard.

What do you think would happen if we shine the light on water in the flask?

*Remember: the beam should be as straight and undivergent as possible. You can achieve this by using a convex lens or two pieces of cardboard with medium-sized holes cut in them.

If you shine the light on the flask, will the spectrum be transmitted through the water and the glass and come out on the table or floor?
Try it with the flask and "turkey light box" provided in the lab.

What did you find? Did you see the "rainbow" on the table?
If the spectrum could not be seen on the side of the flask opposite the light, where might the rainbow be forming?

How could you test your hypothesis?

How would you explain what you observed, based on what you've learned in the Light & Optics Module?

Rainbows Explained

Back to Optics in Nature Page

Light & Optics
Submodule 4
PiN Homepage

Double Rainbows
Inside Rainbows

Copyright &copy1995
Steve Beeson, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287