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Ray Optics and Optical Instruments — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsRay Optics and Optical Instruments1 Mark
Question
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: It is possible to eliminate dispersion by combining two prism of same refracting angles but of different materials.
Reason: The angular dispersion does not depend on refractive index of the material of the prism.
✓

Answer

  1. A is false and R is also false.

Explanation:

For a prism of small angle A, the angular dispersion produced $=(\mu_\text{v}-\mu_\text{R})\text{A}.$ This can be cancelled by a second prism of angle A' made of different material such that $(\mu'_\text{v}-\mu'_\text{R})\text{A}'=(\mu_\text{v}-\mu_\text{R})\text{A}.$

If only A = A', then the dispersion produced by one prism cannot be cancelled by the dispersion produced by the other prism because,

$(\mu'_\text{v}-\mu'_\text{R})\text{A}'=(\mu_\text{v}-\mu_\text{R})\text{A}$ for different materials Therefore in order to eliminate dispersion by combining two prism they should have same refracting angle and made of same material.

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