When light changes direction when it passes from one material into another, it is said to be refracted. Refraction can be observed by putting a pencil in a glass of water and looking at it from an angle above the rim. The pencil appears to be in two separate pieces, above and below the surface of the water. Refraction is caused by light travelling more slowly in water than in air. Many everyday experiences with light, like the sparkle of a diamond and rainbows are caused by refraction.

Refraction through a prism It was Sir Isaac Newton who first conducted an experiment to demonstrate that white light was really made up of a number of colors. He used a glass triangular prism and a beam of light coming through a small hole in a shutter on his window. He was able to project an array of colors onto the wall and identified them as the seven distinct colors that we know as the visible spectrum. He went further though to demonstrate that each color was indeed a separate part of the spectrum. To do this he placed another triangular prism in the line of each color and found that there was no more color change.

When the Sun shines from behind, and there are rain clouds ahead, you can sometimes see a rainbow. This is caused by white light being refracted by the raindrops, but this time being reflected back to the eye as well. The colors of the rainbow are red, orange, yellow, green, blue, indigo and violet. These colors can be remembered by the first letters of a sentence like: Read over your good book in vain. There are other sentences too, and you might know different ways of remembering the sequence.

Chromatic aberration
Because lenses work by refracting light, they also bend different colors through slightly different angles. This means that unwanted colors sometimes appear at the edges of bright parts of the image. Quality cameras and telescopes overcome this aberration by using combinations of lenses to counteract the effect - these are called achromatic lenses. This same chromatic aberration can also be seen at the edges of bevelled mirrors.