Dispersion of light by a prism


 
 
Concept Explanation
 

Dispersion of Light by a Prism

Dispersion Of Light By A Prism: As with any transparent material, lights of different colours travel at different speeds in the material of a prism. Hence, the refractive index of the material of a prism is different for different colours of light. So, when white light enters a prism, its different components (lights of different colours) bend by different amounts. This causes the components to separate out, or split. While emerging from the prism, they undergo a second refraction and bend further, increasing the separation between the colours. In this way, white light gets split into its component colours on passing through a prism.

Note that the component colours of white light bend by different amounts for all refractions. So, why does not light get split while passing through a rectangular slab? Light bends by equal angles but in opposite directions at the parallel faces of a rectangular slab. So, the components of light that split at the first refraction bend back and recombine to give white light after the second refraction.

In the band of colours obtained we can distinguish seven main, or prominent, colours. These are violet, indigo, blue, green, yellow, orange and red, in sequence. This sequence of colours can be remembered as VIBGYOR, which is formed by the first letters of the names of the colours. Whenever white light is split, its coloured components appear in this sequence. You see these colours in a rainbow too. When white light is dispersed, violet light deviates the most and red light deviates the least. This means, the refractive index of the prism material is the largest for violet and the least for red. The collection of coloured components produced by splitting light is called spectrum.

In the activity described above, white light is dispersed into its coloured components. We can recombine these components to produce white light. For this, take two similar prisms and keep the second one inverted with respect to the first. Let a narrow beam of white light fall on the combination of the prisms, and let the emergent light can be recombined by passing them through a fall on a wall. You will get a white patch prism, kept inverted with respect to the first on the wall.

The first prism splits white light into its coloured components. When these separated components pass through the second prism, they recombine to form white light. The second prism was kept inverted with respect to the first to recombine the components of white light.

Sample Questions
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Question : 1

When white light passes through a triangular prism, why does violet light deviate more than red light?

Right Option : B
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Question : 2

large V_V, V_R, V_G are the velocities of a violet, red and green light respectively passing through a prism after the dispersion of white light. Which among the following is a correct relation ?

Right Option : D
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Question : 3

The dispersion of light when it passes through a prism shows that ____________

Right Option : C
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