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Find the focal length of the lens used t...

Find the focal length of the lens used to focus an image of distant object on focal plane of the lens if the radius of central bright dot is `5 mum`, when it illuminated by ray of wavelength 400 nm and the radius of the circular aperture is 2.5 cm.

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`r_0~~(0.61lambdaf)/a`
`r_0=5xx10^(-6)` m
`lambda=400xx10^(-9)` m
a=2.5 cm
f`~~` 0.512
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