The Physics of Rainbows

By contributing writer Marci

I can’t think of anything that stops young and old in their tracks and makes them smile at the wonder of creation quite like rainbows. They are a welcome bright spot on a rainy day and a sign of hope for all.

Rainbows, in all their beauty, can be used to teach physics, particularly the physics of light. I guess that makes it the physics of rainbows!

What is light? (And what does it have to do with color?)

Light is an array of electromagnetic waves that are visible to the eye. The varying wavelengths of light create color. When waves of light hit an object, certain wavelengths are absorbed and others are reflected. We see those reflected wavelengths of light as color. For example, if you see green leaves on a tree, the color being reflected by the leaves is green.

When we see the color black, that object is absorbing almost all the wavelengths of light. (That is why black shirts and cars get so hot in the summertime – they are absorbing all those hot rays of sunlight.)

White objects are actually a combination of all colors. They reflect all the wavelengths of light.

How does light make a rainbow?

When sunlight passes through raindrops in the sky, the drops act like tiny prisms that bend, or refract, the sunlight. This bending causes the light to separate into its separate wavelengths of color which reflect back to our eyes causing us to see the rainbow.

When do rainbows occur?

Rainbows are seen when sun is shining through raindrops and the viewer is between the sun and those raindrops. The sun needs to be low in the sky (morning or early evening) for this to happen.

Can you make your own rainbow?

Yes! All you need is water and light.

1. Garden Hose Rainbow – On a sunny day, spray water from a garden hose into the air. Make sure you are between the water and the sun. A rainbow will appear.
2. Use A Prism – Get a prism and a piece of white paper. Allow sunlight or light from a flashlight to shine through the prism. Position the paper so that the light from the prism shines on the paper. This light should be in the form of a rainbow!
3. Make A Water Prism – Fill a glass ¾ full of water. Place the glass on a white piece of paper. Set the glass where sunlight or a flashlight can shine through it. The water in the glass will act like a prism and bend the light into a rainbow on the white paper. The angle of light can be adjusted until you see the rainbow.

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