Question

Light of 5000 Å is incident on a circular hole of radius 1 cm. How many half period zones are contained in the circle if the screen is placed at a distance of 1 m?

(a) 20

(b) 200

(c) 2000

(d) 20000

This question was addressed to me during an interview for a job.

The above asked question is from Fresnel Diffraction in portion Optics of Engineering Physics II

Answer 1

Right answer is (d) 20000

Explanation: In this case, λ = 5000 Å = 5 X 10^-5cm, b = 1m = 100cm

Therefore, Number of half period zones = \(\frac{1}{λ}\)

= 1/5 X 10^-5

= 20000.