# Interference of electromagnetic waves

1. Jun 7, 2012

### freddyfish

I haven't been able to find any literature that answers this question in a pleasing way. Also, all sources on the internet are contradictory.

If electromagnetic waves (out of phase since they are emitted by the gaseous atoms in discharge tube) sent through a diffraction gitter interfered with each other there would not be a symmetric interference pattern. When performing some diffraction experiment, a symmetric pattern obviously arises at the screen though. The same frequency is obviously not enough for the waves to interfere, so I wonder:

What are the criteria for electromagnetic waves to interfere with each other? None of my teachers has been able to answer this question properly and I have recieved the most unreasonable answers you can possably imagine. :p

Thanks //F

2. Jun 7, 2012

### PhilDSP

Hi freddyfish,

Since EM waves obey the superposition principle (their amplitudes add linearly) waves from different sources pretty much always interfere with each other as they traverse the same location at the same time.

What you are probably most interested in though is when you can observe obvious patterns of constructive and destructive interference. Those are more easily observable if the frequencies lie near each other (or near multiples of each other) and their phase differences are closer towards zero or 180 degrees and when either the wave trajectories are parallel or anti-parallel (or possibly near to perpendicular).

Whether the pattern is symmetric or not I believe is determined by the particular phase differences between the frequency components of both waves. Monochromatic interfering waves near each other in frequency shouldn't produce an asymmetrical pattern (unless they are undergoing refraction or are impinging at an angle on each other).

Last edited: Jun 7, 2012
3. Jun 7, 2012

### Rap

The EM waves emitted by atoms in a gas are not "out of phase". They have random phases, and there is a difference. They are also emitted in random directions. Two EM waves cannot cancel each other out (interfere destructively) or add together (interfere constructively) unless they are travelling in practically the same direction. So if you pick two atoms in the gas, chances are they will not be interfering with each other, because chances are they will not be emitting in the same direction. When you look at a gas of emitting atoms, all you see are the atoms that happen to be emitting in more or less your direction. Even then, since the phases are random, there will not be complete cancelling. There will be some constructive interference, some destructive interference.