What happens to a white light when it passes through a prism and why?

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What happens to a white light when it passes through a prism and why?
White light disperses through a prism. (Thinkstock Images/Comstock/Getty Images)

Visible light, like the light of the sun, or light from a light bulb or a torch, is called “white light,” even though it isn’t actually white. A prism is a triangular solid piece of glass, which looks like a transparent pitched roof. When white light hits a prism, strange things happen.

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Light bends when it passes from one medium to another. This is caused by the different density of the two substances. You can see this effect if you plunge your arm into a stream. Your arm will seem to bend unnaturally at the point where it passes into the water. This is an optical illusion caused by “refraction,” which is the bending of light. When light passes from air to glass and back again through a prism it bends twice.


The sloped sides of the prism cause the light entering it to bend at a greater rate than it would if it passed through a perpendicular piece of glass. This accentuates the effects of refraction and causes another process, called dispersion.


White light is actually made up of seven different colours, called the “spectrum.” In their natural state, bound together, the colours cancel each other out. However, the process of dispersion, caused by the angles of the prism, splits out these component colours, resulting in the projection of a rainbow on the other side of the prism.


Different colours have different wavelengths and so travel at different speeds. The colours with shorter wavelengths have higher frequency and these travel slower through the prism. In the spectrum, violet light has the highest frequency, and so this is bent most to deviate from the original path of the white light. Red light has the lowest frequency and this bends the least. Each of the seven colours has a slightly different frequency and so have a slightly different angle of deviation. This results in all the colours fanning out as they exit. The order of frequency of the seven colours of the spectrum, from lowest to highest is: red, orange, yellow, green, blue, indigo, violet.


A very well known natural occurrence illustrates the lab experiment with the white light and the glass prism. The order of frequency of the seven colours of the spectrum should give you a clue to what that phenomenon is. Schoolchildren are taught to remember the colours of the rainbow with the mnemonic, “Richard of York gave battle in vain.” Reduce this phrase to its initials to get R O Y G B I V. These are the seven colours of the spectrum in the exact order they appear in the prism experiment: red, orange, yellow, green, blue, indigo and violet. Rainbows appear when sun shines on a rainy day. Just like the glass prism, raindrops contain a different density (water) to the air around them and their sides are angled. Sunlight refracts through raindrops and creates the rainbow. It is this natural phenomenon that the lab experiment recreates.

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