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    Question

    A prism has a refractive index of 1.5 and a refracting

    angle of 60°.One of its refracting faces is polished to act like a mirror. A monochromatic beam of light is incident on the other face. For the light beam to retrace its path , the angle of incidence at the first face should be:
    A 63.8° Correct Answer Incorrect Answer
    B 48.6° Correct Answer Incorrect Answer
    C 46.8° Correct Answer Incorrect Answer
    D 64.8° Correct Answer Incorrect Answer

    Solution

    Given:

    • Refractive index of prism, ÎĽ=1.5
    • Refracting angle of prism, A= 60°
    • One refracting face is silvered (acts like a mirror).
    • A monochromatic beam is incident on the other refracting face .
    Inside the prism, the light must fall normally (at 90° ) on the silvered face for exact retracing after reflection. Then after reflecting and passing out through the first face, it should retrace exactly. This demands that inside the prism, the ray must make an angle of A/2 with the normal.

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