When light strikes a transparent surface at an angle, its speed reduces and the rays of light bend slightly. The ‘white’ light we see is actually a mixture of colours: red, orange, yellow, green, blue, indigo and violet; each of these colours is bent by a different amount. So when a beam of white light passes through a glass prism, the light coming out the other side consists of bands of these colours. You can often see this banded effect around the bevelled edges of a mirror.
This process of refraction, or bending, of light also takes place in raindrops when sunlight strikes them, which is why you see the colours of the spectrum in a rainbow.
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