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A convex lens of diameter 8.0 cm is used...

A convex lens of diameter 8.0 cm is used to focus a parallel beam of light of wavelength 620 nm. 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 ?

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The correct Answer is:
A, C
`lamda =620nm =620xx10^-9m,
D=20cm=20xx10^-2m,
d=8cm=8xx10^-2m`
`:.R=1.22xx(620xx10^-9xx20xx10^-2)/(8xx10^-2)`
`=1891xx10^-9`
`=1.9xx10^-6m`
So, diameter `=2R=3.8xx10^-6m`
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