Question

For a plane transmission grating with 5000 lines/cm what are the the absent order spectra if the width of the opacity is twice that of the frequency?

(a) 1^st, 3^rd, 5^th, …

(b) 2^nd, 4^th, 6^th, …

(c) 3^rd, 6^th, 9^th, …

(d) 4^th, 8^th, 12^th,…

The question was asked in an interview for internship.

This intriguing question comes from Plane Diffraction Grating in chapter Diffraction of Engineering Physics II

Answer 1

Correct answer is (c) 3^rd, 6^th, 9^th, …

Easy explanation: Now, for the order n to be missing

\(\frac{a+b}{a}=\frac{n}{M}\)

n/m = 1 + b/a

n/m = 3

n = 3m

Hence, the missing order spectra are 3^rd, 6^th, 9^th, …