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A thin prism having refracting angle 10° is made of glass of refractive index 1.42. This prism is combined with another thin prism of glass of refractive index 1.7. This combination produces dispersion without deviation. The refracting angle of second prism should be

1. 6°
2. 8°
3. 10°
4. 4°

Correct answer: 6°

Solution

For dispersion without deviation, the net deviation caused by the two prisms must cancel out. The deviation caused by a prism is given by (μ - 1)A, where μ is the refractive index and A is the refracting angle. Setting the deviations equal in magnitude but opposite in direction, we solve for the refracting angle of the second prism, which comes out to be 6°.

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