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In a biprism experiment, a mica sheet of thickness 7 microns and refractive index 1.6 is introduced in the path of one of the interfering beams. The central fringe then shifts to the former position of the seventh bright fringe. Find the wavelength of the light used.

1. 5000 angstrom
2. 6000 angstrom
3. 6500 angstrom
4. 4500 angstrom

Correct answer: 6000 angstrom

Solution

A transparent sheet of thickness t and index mu adds an optical path (mu - 1)t. The central fringe shifts by n fringes when (mu - 1)t = n*lambda. Solving for lambda with mu = 1.6, t = 7 microns, n = 7 gives lambda = 6000 angstrom.

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