A laser beam has a wavelength of `7 xx 10^(-7) m` and aperure `10^(-2)m`. The beam is sent to moon at a distance of `4 xx 10^(5) km` from earth. Find the angular spread and areal spread of the beam on reaching the moon.
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Here, `lambda = 7 xx 10^(-7) m, a = 10^(-2)m` `D = 4 xx 10^(5) km = 4 xx 10^(8)m` For diffraction at a circular aperture, angular spread `= theta = (1.22 lamda)/(d) = (1.22 xx 7 xx 10^(-7))/(10^(-2))` `= 8.54 xx 10^(-5) radian` Areal spread `= (D theta)^(2)` `= (4 xx 10^(8) xx 8.54 xx 10^(-5))^(2) = 1.197 xx 10^(9)m^(2)`
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