Complete the sentence: A mixture of colours shows up when white light hits a r______.

using a prism. Due to change in refractive index , it changes depending on the wavelength of the light. When you pass white light through a prism, each of the wavelengths that make up the white light is bent by the prism a different amount

The answer is raindrop. In this respect a raindrop acts rather like a prism and due to each colour of light ( all of which added makes white light) having different wavelength they get refracted or bent by slightly different angles giving us the colours of the rainbow.

Raindrop. The raindrop acts like a prism and splits the light into the colours of the visible spectrum. The splitting is called dispersion. The splitting is caused by refraction.

That's not the whole story though. After entering the raindtop, the light is reflected off the back surface. It then travels out of the front of the drop, refracting again at the boundary. If, when this light leaves the raindrop, it happens to travel to your eyes, you see the familiar bow in the sky. What this means is that nobody sees the same rainbow as you. A friend stood next to you sees the light that has been refracted in their direction.

Ever heard of the famous double rainbow? They are formed when the light reflects twice inside the raindrop. Have you ever noticed that the colours in the second rainbow are reversed? The diagram above shows why.

Other effects you may notice when you see a really good rainbow are supernumary bows (green and magenta bows underneath the other bows. These are caused by a process called interference), Alexander's dark band (a dark region between the two bows) and that the area under the bows is very bright.

Look here for some other light effects you may see in the atmosphere. I would really like to see the spooky Brocken Spectre. Light really is fantastic.