Question

Find the missing order for a double-slit Fraunhofer Diffraction pattern if the slit widths are 0.2 mm separated by 0.6 mm.

(a) 1^st, 5^th, 9^th, ….

(b) 2^nd, 6^th, 10^th, …

(c) 3^rd, 7^th, 11^th, ….

(d) 4^th, 8^th, 12^th, …

The question was posed to me by my school principal while I was bunking the class.

The origin of the question is Fresnel and Fraunhofer Diffraction in chapter Diffraction of Engineering Physics II

Answer 1

Correct option is (d) 4^th, 8^th, 12^th, …

Easy explanation: We know, for interference maxima (e + d) sin θ = ±nλ and diffraction minima e sinθ = ±mλ.

Therefore,      (a+b)/a=n/m

(0.2+0.6)/0.2=n/m

n = 4 m

As m = 1, 2, 3, …. Therefore, 4^th, 8^th, 12^th, …. order interference maximum will be missing.