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In a Fresnel biprism experiment the source-to-screen distance is 1 m and the source-to-biprism distance is 10 cm. Light of wavelength 6000 angstrom is used, the measured fringe width is 0.03 cm, and the refracting angle of the biprism is 1 deg. Find the refractive index of the biprism material.

1. 1.57
2. 1.33
3. 1.50
4. 1.68

Correct answer: 1.57

Solution

From beta = lambda*D/d, find d = 2*10⁻³ m. Then using d = 2*a*(mu-1)*alpha with a = 0.10 m and alpha = 1 deg (0.01745 rad) gives (mu - 1) ~ 0.573, so mu ~ 1.57.

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