I obsessed about rainbows as a little girl. Rainbow stickers, rainbow shoelaces, and who remembers Rainbow Brite? I drew rainbows over and over, red, orange, yellow, green, blue, purple, with puffy white clouds on each end. They adorned my book covers in elementary school and hung in my window as shrinky-dinks. Rainbows are one of the most beloved icons of a girl’s childhood, right up there with hearts and unicorns. As a young girl, rainbows were magical. Even today it’s a little exciting to see one emerge after it rains. Rainbows are rare and fleeting, so catching a glimpse of one is truly special. Unless you live in Hawaii. On my honeymoon there were rainbows in the sky everyday. It is the rainbow state.
My vague understanding of rainbows is that rain + sun = rainbow and something about light and prisms. So, I decided to find out what causes the spectacular array of color in the sky.
Rainbows appear through a process called refraction. This is when light bends as it passes from one medium to another. In the case of rainbows, light is passing from the air into rain, and then back into the air once again. The light bends because it passes through water at a slower speed than it passes through air.
Another way to illustrate refraction is when you are pushing one of those giant car carts at the grocery store and one of the wheels hits the corner of a Goldfish display. One wheel slows down while the other side of the cart continues at full speed, causing the direction of the cart to bend (once momentum builds in those massive carts, they are impossible to stop and I have taken down more than one display. But I digress...). The cart travels at one speed on the floor and at another speed when pushing through a cardboard display.
Why do we see so many colors in rainbow? Light is actually composed of different colored light: red, orange, yellow, green, blue, indigo, and violet (remember Roy G. Biv?). The colors have different wavelengths and thus bend at different angles when refracted. Red has the longest wavelength and bends the least, whereas violet has the shortest wavelength and bends the most. That is why the colors of a rainbow always appear in the same order.
How is color a wavelength, you ask? I asked myself the same question. Visible light lies on the Electromagnetic Spectrum, along with radio waves and infra-red light. Visible light is the portion of the spectrum that is visible to the naked eye. Below is a diagram to illustrate:
So, when sunlight passes through a raindrop or another substance such as a prism, the light bends. The various colors that make up light bend at different angles, creating a beautiful arc of color.
Why do rainbows form an arc? Rainbows technically are full circles but we don’t see the circle because of the earth. The extent of the rainbow that we see depends on the position of the sun. The higher the sun, the less rainbow we see. The longest rainbow will be when the sun is close to the horizon, such as at sunset, and the rainbow will be a semi-circle over the sun.
So the next time your children shriek with delight, “Look! A rainbow!”, you will be able to help them understand how the rainbow came to be. You can make a rainbow in your house in the experiment below.
Experiment:
Click here to learn how you and your children can make a rainbow with water, a mirror, and a flashlight.
Sources:
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