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Question:

A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aperture is illuminated normally by a parallel beam of wavelength 5 × 10⁻⁵ cm. The distance of the first dark band of the diffraction pattern from the centre of the screen is:

0.15 cm

0.10 cm

0.25 cm

0.20 cm

Solution:

f = D = 60 cm
For first minima, y = λD/a = (5 × 10⁻⁵ cm × 60 cm) / (2 × 10⁻² cm) = 0.15 cm