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Estimate how large can be the aperture o...

Estimate how large can be the aperture opening to work with laws of ray optics using a monochromatic light of wavelength 450 nm, to a distance of around 20 m.

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Now, here we are given Fresnel distance equal to 20 m , `lambda=450 nm`, we have to estimate value of a.
Putting `Z_(F)=(a^(2))/(lambda)`
`a=sqrt(Z_(F)lambda)`
`=3xx10^(-3)m` or 3 mm
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