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A spectrum of colours: the dispersion of light

Physics Narrative for 11-14 Supporting Physics Teaching

A spectrum of colours

In 1666 Sir Isaac Newton studied the effect of passing a beam of light through a triangular glass prism. He completely darkened his study and made a small hole in the window shutters to let in a fine beam of sunlight. He then held a glass prism in the beam and noticed that a brilliant spectrum of colours, from red to violet, formed on the wall at the other side of the room. In his own words (taken from a letter to the Royal Society): It was at first a very pleasing divertissement, to view the vivid and intense colours produced thereby.

Newton concluded from this simple experiment that the light from the Sun, or white light, as he referred to it, is made up from a whole spectrum of different colours. How can we explain the production of the spectrum?

As you might expect the sunlight is refracted as it first passes from air into the glass prism. The light is refracted towards the normal. As the light leaves the prism it is refracted away from the normal. However, the different colours which constitute the white light are not refracted by the same amount.

The red light is refracted least.

The violet light is refracted most.

Light is refracted

With a rectangular glass block, the light beam is refracted first one way and then back in the opposite direction, so that the emerging beam is parallel to the incident beam. With the triangular block, the geometry of the prism is such that there is some separation of the different colours of light at the first face and then further separation at the second.

As a result the red light lands at a different point on the screen to the violet light and in between we can see the full spectrum of colours. The white light is dispersed into its constituent colours and the process is therefore referred to as dispersion.

In Newton's own words: The light tending towards one end of the Image, did suffer a Refraction considerably greater then the light tending to the other. And so the true cause of … that image was detected to be no other, than that Light consists of Rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

Violet light is refracted more than red light. But why does this happen? Following the previous explanation for refraction, we can say that the violet light must be slowed down more than the red light as it enters the glass. The violet light therefore swings round more as it enters the glass and is refracted more.

Returning to the concept of frequency introduced in episode 01, we can say generally that higher frequencies of light are refracted more than lower frequencies. But that's still only a rule of thumb, not something that could really be an explanation. If you want more, you'll have to delve deeper: there is more in the SPT: Radiations and radiating topic.

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