Ncert Exercises & Solutions

9.23:
You are given prisms made of crown glass and flint glass with a wide variety of angles.
Suggest a combination of prisms which will
(i) deviate a pencil of white light without much dispersion,
(ii) disperse (and displace) a pencil of white light without much deviation.

Hint: Recall the concept of dispersion and deviation.

Step 1: Find the combination of two prisms such that the dispersion is zero.
The two prisms should be arranged such that the dispersion produced by one prism is canceled by the second prism. This way is called the direct vision prism. The dispersion of the crown glass prism is less than the flint glass prism. So, the flint glass prism of a smaller angle will be suitable with a crown glass prism of a larger angle.

Step 2: Find the combination of two prisms such that the deviation is minimum.
To disperse without any deviation, the flint glass prism of greater angle close to each other in opposite directions is required so that the deviation is equal and opposite. So, in the final combination, the flint glass prism will have a smaller angle than the crown glass prism because of their refractive indices.

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