Refraction And Dispersion Of Light Through A Prism

Rainbow aren’t they beautiful? Have you ever wonder How rainbow is created? It is a simple physical phenomenon called dispersion of light. So, what is the dispersion of light? Splitting of white light into seven constituent colors when passed through the prism. Prism is an optical element which is transparent with flat, polished surfaces that refract light. Change in the direction of propagation of light when passed through the different medium is called Refraction of light.


This article deals with the following topics regarding the 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.
The simplest way to explain dispersion is through dispersion in the prism.

Refraction Of Light Through Prism

How is light refracted through a glass prism?
When light travels from one medium to another, the speed of its propagation changes, as a result, it ‘bends’ or is ‘refracted’. Now when light passes through a prism, it is refracted towards the base of the triangle. Refraction Of Light Through Prism is well illustrated in the diagram given above.

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

Visible Light Spectrum

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 the path that is different from each other, giving a dispersed effect.

Prism Experiment

What is the significance of the dispersion of light into its spectrum of colours?
Newton was the first to conduct this experiment on passing light through a prism. He let sunlight pass through the prism expecting to see the 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 colour (therefore only one wavelength of light) of light to pass through the prism. Obviously, the ray of light was refracted and didn’t undergo any further dispersion.

Therefore, he realized that different colours 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.

Hope you have understood what is dispersion of light? What is refraction of light? Dispersion of light in prism, related theory and Learn more about Optics do click on the links mentioned below.

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Practise This Question

Angle of minimum deviation for a prism of refractive index 1.5 is equal to the angle of prism. The angle of prism is (cos41=0.75)