If electromagnetic (light) waves from the sun are (essentially) randomly polarized, then why doesn't their superposition average out and cancel to darkness?

Question

anonymous · Answer

BUT I DONT THINK SUN IS A COHERENT SOURCE OF LIGHT.

anonymous · Answer

What do you mean by "coherent"?

anonymous · Answer

Be careful not to confuse random polarization with random phase.  Two light waves with orthogonal polarization don't cancel each other out, that happens when two light waves are 180 degrees out of phase.

anonymous · Answer

Hmm interesting...  so the entire sun emits light entirely in phase?

eyust707 · Answer

http://www.bautforum.com/showthread.php/46109-Why-doesn-t-light-from-distant-objects-destructively-interfere-with-itself

anonymous · Answer

No, that's not what I meant.  I was just trying to say that interference is not caused by relative polarization between electromagnetic waves.  

To answer your question, for electromagnetic waves to destructively interfere they need to be a) out of phase and b) spatially and temporally coherent, i.e. if they're out of phase at time t1, they will be out of phase at time t2, and if they're out of phase at location x, they will be out of phase at location y.  You are correct in that the phases of the electromagnetic waves are random, but because the sun does not produce monochromatic light, there is no coherence and therefore no cancellation effect.  You'll notice that if you do the famed double-slit experiment with an incandescent light bulb rather than a gas discharge tube or a laser, it won't "work" properly insofar as you won't see the pretty interference patterns.

anonymous · Answer

Ah okay!  This makes sense now.  Thanks for all of the responses!

vincent-lyon.fr · Answer

I'm late and the answer was already given, but you can also explain why this can not happen considering conservation of energy.

When photons are emitted, whether random or not, coherent or not, their energies will always add up, even when interferences are observed.

In order to completely cancel out, 2 photons should be emitted at the same time, in the same direction, with the same polarization. All this means they should be emitted at the same time by the same atom, which is not possible.