Many of us have seen rainbows in the sky once the sun starts shining again after a spell of rain. For us to see a rainbow, the conditions need to be just right.

We need some water droplets 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 droplets.

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

Waves of light

You wouldn't know it to look at it, but light travels in waves, like waves moving across the ocean. Each of the colors in the rainbow have what we call a different "wavelength".

This means that the distance between the tops of the wave is a different length for each color. The colors, from violet with the shortest wavelength to red with the longest, are called the "visible spectrum".

Raindrops look more like little balls than the teardrop shapes we often draw. When light hits one of these little balls of water, the light can change direction. We call this "refraction".

Each of the different wavelengths is refracted by a slightly different amount. If the light hits the raindrop at the right angle, the refraction separates the wavelengths out into their different colors. As lots of light is refracted through lots of raindrops, we see these colors as a rainbow in the sky. The order the colors come in is set by how long their wavelength is.

When we learn about the rainbow, we are taught that there are seven colors: red, orange, yellow, green, blue, indigo and violet. But this isn't strictly true.

Colors in the rainbow

The different colors blend into each other, and it is difficult to tell where one color ends and another begins. There are other colors in between them, where they mix – like turquoise between blue and green.

Blue and green are next to each other in the color spectrum, which is why we can see turquoise where they blend into each other. Some colors, though, are mixes of colors that aren't next to each other in the spectrum.

Brown, for instance, is a mix of red and green. But the red and green bands in the rainbow aren't next to each other, so we don't see them mix to make brown. The same is the case for many other colors that are mixtures – if the color bands in the rainbow don't overlap, then they can't mix.

But there are two colors we would never see in a rainbow - black and white. Black is the absence of color – it's what we see when there's no light at all.

On the other hand, white is a combination of all the colors together. When light is refracted by raindrops, it separates the white light out into the visible spectrum, meaning it is no longer white. Grey is a mix of black and white, and as we can't ever see black and white in a rainbow, we also can't see colors made by mixing them.

Next time you see a rainbow, look out for how many colors you can spot in it – and the colors that you can't see.The Conversation

James Rawlings, Hourly Paid Lecturer in Physics, Nottingham Trent University

This article is republished from The Conversation under a Creative Commons license. Read the original article.