Question

If the separation between the two slits in Double Slit Fraunhofer Diffraction is changed, what change will be observed in the diffraction pattern?

(a) The fringe length will increase

(b) The fringe length will decrease

(c) Fringes will be colored

(d) No change

I had been asked this question in an interview.

This intriguing question comes from Fraunhofer Diffraction topic in division Optics of Engineering Physics II

Answer 1

Correct choice is (d) No change

To elaborate: The separation between the two slits only affects the interference pattern in Double Slit Fraunhofer Diffraction. The diffraction pattern does not change.

For Diffraction, e sin θ = ±mλ

Where e = Width of slits

 m = Any integer

For interference, (e + d) sin θ = ±nλ

Where e = Width of slits

d = Separation between the two slits

m = Any integer

Hence, there is no change in the Diffraction Pattern.