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Question

# A convex lens of diameter 8 cm is used to focus to parallel beam of light of wavelength 6200 ˙A. If the light be focused at a distance of 20 cm from the lens. What would be the radius of the central bright spot formed?

A
2.89 μm
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B
1.5 μm
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C
1.89 μm
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D
1.79 μm
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Solution

## The correct option is C 1.89 μm The angular spread of the central bright spot, θ=1.22λd=1.22×6200×10−108×10−2 ∴θ=9.455×10−6 As θ is small, from figure θ=rD ⇒r=θD=9.455×10−6×20×10−2 m =1.89×10−6 m=1.89 μm Hence, the correct option is (C). Note; The diffraction pattern due to circular aperture consists of a bright circular disc as a central maximum, which is called Airy's disc.

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