Question

In three to five sentences, explain why two slightly separated coherent sources of circular waves results in an alternating pattern of “bright” and “dark” bands on a screen held some reasonable distance away. Assume the two sources are coherently in phase (e.g., both are at a peak at a particular instant) at the same frequency.

Answer 1

Let us consider a setup where two coherent wave sources are separated slightly and a screen is placed at a distance from the sources. The sources are coherent means that the waves produced by the sources have same frequency and constant phase difference. Now as the waves move outward, they interfere. There will be constructive interference where the waves are in phase and we shall observe bright region on the screen. There will be destructive interference where the waves are 180⁰ out of phase and we shall observe a dark region on the screen. Thus we observe a pattern of bright and dark bands on the screen.