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Monochromatic light of wavelength 500 nm strikes a slit of width 10 micrometre at an angle of incidence 30 degrees to the slit's normal. Find the angular positions of the first diffraction minima on the two sides of the central Fraunhofer maximum.

1. 33 deg and 27 deg
2. 25 deg and 35 deg
3. 36 deg and 24 deg
4. 28 deg and 32 deg

Correct answer: 33 deg and 27 deg

Solution

For oblique incidence the first minima obey a*(sin(theta) - sin(i)) = +/- lambda. Here lambda/a = 500e-9/10e-6 = 0.05. sin(i) = sin(30) = 0.5. So sin(theta) = 0.5 +/- 0.05 = 0.55 or 0.45, giving theta ~= 33.4 deg and 26.7 deg. Hence the first minima lie at about 33 deg and 27 deg.

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