Question

Identify the condition which is not necessary for two light waves to be coherent.

(a) The two waves must be continuous

(b) The two waves should be of the same frequency or wavelength

(c) They should have a constant or zero phases difference

(d) They two light sources should be narrow

I had been asked this question during an online interview.

My question is based upon Wave Optics in division Wave Optics of Physics – Class 12

Answer 1

Correct answer is (d) They two light sources should be narrow

The best explanation: The essential conditions for two light waves to be coherent includes that the two waves must be continuous, the two waves should be of the same frequency or wavelength, and they should have a constant or zero phases difference. So, the unnecessary condition for two light waves to be coherent is the statement that the sources should be narrow.