Do Light Interference Fringes Change?

Most recent answer: 01/07/2013

Q:
In Young's Double Slits experiment, why are the interference fringes stay with constant brightness at all times? It is understandable from superposition principle that when crest meets crest the result is double crest at one instant. But the next instant the wave amplitudes will have changed, yet the resultant amplitude remains the same at the same place in space.
- Cugan Nathan (age 17)
London, UK
A:
If the delay is right so that crest meets crest, a half wave period later then trough will meet trough. So though the electric field oscillates the interference pattern doesn't shift. It's true that for linearly polarized light of some frequency the magnitude of the electric field oscillates between a maximum value and zero, as does any sine wave. This is true whether or not you think of the wave as a the sum of two contributions from different sources. However, any intensity measurement averages over many periods of the wave (each about 10-15 s) of the wave, so you don't see a flickering intensity.

Mike W.

(published on 01/07/2013)