Answer to Question #162498 in Optics for Fletcher

Question

Explain why two flashlights held close together do not produce an interference pattern on a distant screen

Answer 1

Condition for sustained interference is two sources should be coherent, i.e., they should vibrate in the same phase and there should be a constant phase difference between them.

Two sources made from single source shows coherence. But two different sources never show coherence.

Therefore two flashlights do not produce interference pattern on a distant screen.