Question

 A beam of parallel light of wavelength 6000 Å is incident normally on a 1 rupee  coin of diameter 3 cm. If an observation screen is placed at a distance of 1 m from  the coin, how many Fresnel zones would be obstructed by it (the coin)? For what  separation between the coin and the screen, only 5 zones would be cut off?

Answer 1

1)

$m=\frac{d^2}{4\lambda a }=\frac{0.03^2}{4\cdot 6\cdot 10^{-7}\cdot1}=375,$

2)

$a'=\frac{d^2}{4\lambda m'}=\frac{0.03^2}{4\cdot 6\cdot 10^{-7}\cdot 5}=75~\text m.$