Refraction and Dispersion of Light through a Prism

Refraction-and-Dispersion-of-Light-through-a-Prism

This article deals with the following topics regarding dispersion of light through a prism.

  1. What is dispersion of light?
  2. How is light refracted through a glass prism?
  3. Why does light split into its spectrum of colors in a prism but not in a glass slab?
  4. What is the significance of the dispersion of light into its spectrum of colors?

What is dispersion of light?

When white light is passed through a glass prism it splits into its spectrum of colors (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colors is termed as dispersion.

How is light refracted through a glass prism?

When light travels from one medium to another, the speed of its propagation changes, which is why it ‘bends’ or is ‘refracted’. Now when light passes through a prism, it is refracted towards the base of the triangle.

The different colors in the spectrum of light have different wavelengths. Therefore, the speed with which they all bend varies depending on this wavelength, where violet bends the most, having the shortest wavelength and red bends the least, having the longest wavelength.

Because of this, dispersion white light into its spectrum of colors takes place when refracted through a prism.

Why does light split into its spectrum of colors in a prism but not in a glass slab?

Actually, light does disperse into its spectrum of colors in a glass slab as well. We can see this if we observe it in a particular way. Before we proceed you should know something about refractive indices. They aren’t constant. They vary with the frequency of light and hence the wavelength.

Now, for white light to pass through a glass slab or a glass prism, it is refracted not once, but twice. It first travels from air to glass and then from glass to air. At the first instance of refraction, it slows down and at the second it speeds up right?

So what happens in a glass slab? All the light rays slow down and speed up at the same rate because both the surfaces are parallel. And hence, to an observer it would seem as if white light has entered and left the slab. But the case is different in a prism. The surfaces aren’t parallel to each other and so the light rays emerging out of the prism finally follow path that are different from each other, giving a dispersed effect.

What is the significance of the dispersion of light into its spectrum of colors?

Newton was the first to conduct this experiment on passing light through a prism. He let sunlight pass through the prism expecting to see white light on the screen places at the other side but instead he saw the spectrum of light after dispersion. He had a small hunch regarding the significance here, but decided to do something else here to confirm it.

By controlling the size of the inlet, he allowed only one color (therefore only one wavelength of light) of light to pass through the prism. Obviously, the ray of light was refracted and didn’t undergo and further dispersion.

Therefore, he realized that different colors of the spectrum of light bend differently as they have different wavelengths. He made the observation that violet bent the most and red the least because of their shorter and longer wavelengths respectively.

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Practise This Question

A prism ABC of angle 30 has its face AC silvered. A ray of light incident at an angle of 45 at the face AB retraces its path after refraction at face AB and reflection at face AC. The refractive index of the material of the prism is