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In a Young's double-slit experiment the slit separation is 0.5 mm and the interference pattern is seen on a screen 100 cm from the slits. The 9th bright fringe is found 7.5 mm from the centre of the pattern (on one side). What is the wavelength of the light used?

1. 4167 angstrom
2. 5000 angstrom
3. 6000 angstrom
4. 3750 angstrom

Correct answer: 4167 angstrom

Solution

Bright fringes occur at y = n*lambda*D/d. Substituting the 9th fringe position gives the wavelength directly.

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