# Why don't two identical monochromatic lamps form interference patterns?

Monochromatic light of intensity I from two identical lamps is incident on a screen. How will the intensity vary with distance, x, from the center of the screen.

From my knowledge of Physics, I would instantly point this out to be an example of double slit interference where the slit width is negligible, i.e. there is not single slit diffraction effects. Strangely, I found the answer to this question to be a horizontal line, with the intensity 2I.

First of all, how is the graph horizontal, and second of all how is the intensity just double the original intensity? I would have thought that as a result of interference, the amplitude at the center would be twice the amplitude of light from one lamp, and so the intensity from two lamps if four times the intensity from one? One of the answers I came across is that the phase difference is not constant, but how is this the case if the two lamps are identical?

• Apr 19, 2022 at 15:53