Many of us have seen rainbows in the sky once the sun starts

shining again after it rains. For us to see a rainbow, the conditions need to be just right. We need some water drops in the air, like rain or even fog, and we need the Sun to be behind us and quite low to the ground. This is because a rainbow is created by light passing through water drops. 

The light that comes from the sun seems white to us. But the white light we see is made up of a mix of different colors. When the light goes through a raindrop, these colors can separate out. 

Each of the colors in the rainbow has a different "wavelength" (波长). Raindrops look like lttle balls. When light hits one of these lttle balls of water, the light can change direction. We call this "refraction"(折射). Each of the different wavelengths is refracted differently. If the light hits the raindrop at the right place, the refraction separates the wavelengths out into their different colors.

We are taught there are seven colors: red, orange, yellow, green, blue, indigo and violet. But this isn't exactly true. Blue and green are next to each other in the rainbow, which is why we can see turquoise (a mix of blue and green). Brown is a mix of red and green. But they aren't next to each other in the rainbow, so we don't see them mix to make brown.

We would never see black and white in a rainbow. Black is what we see when there' s no light at all. On the other hand, white is a mix of all the colors together. When light is refracted by raindrops, it separates the white light out, meaning it is no longer white.