Rainbow, aren’t they beautiful? Have you ever wondered how a 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 colours 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 a different medium is called refraction of light.
What Is Dispersion of Light?
When white light is passed through a glass prism it splits into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colours 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. The refraction of light through the prism is well illustrated in the diagram given above.
- The different colours in the spectrum of light have different wavelengths. Therefore, the speed with which they all bend varies depending on their 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 colours takes place when refracted through a prism.
Visible Light Spectrum
Why does light split into its spectrum of colours in a prism but not in a glass slab?
Actually, light does disperse into its spectrum of colours 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 in the second it speeds upright?
- So what happens in a glass slab? All the light rays slow down and speed up at the same rate because both 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 placed 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.
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