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# Question 23: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments

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Question 23: NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments

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

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1. Solution:
By using two identical shape prism of crown glass and flint glass kept with their base on opposite sides, we can observe deviation without dispersion or dispersion without deviation.
(a) Deviation without dispersion A white beam incident on crown glass will suffer deviation 6, and angular dispersion Δθ, both

Now the light beam again suffer deviation and dispersion by flint glass prism.

Negative sign shows that two prisms must be placed with their base on opposite sides. Net deviation in this condition

(b) Dispersion without deviation

Here again negative sign shows that two prisms must be placed with their base on opposite sides. Net angular dispersion

Δθ = Δθ1 + Δθ2
Δθ = A1 (μv1 – μR1) + A2 (μv2 – μR2)

Check the complete chapter with solutions.

NCERT Solutions for 12th Class Physics: Chapter 9-Ray Optics and Optical Instruments