Why Doesn't Incident Light Interfere with Reflected Light in a Mirror?

[cepheid]

Say that light is incident on a plane mirror, normal to the surface (i.e. zero degrees from the normal). Why does the reflected light (which heads back toward the source) not interfere with the incident light?

 

[ZapperZ]

Say that light is incident on a plane mirror, normal to the surface (i.e. zero degrees from the normal). Why does the reflected light (which heads back toward the source) not interfere with the incident light?

In some cases, it does! That's why you have standing wave in EM cavities.

It is more difficult when you try to do it with a common laser source, for instance, because for it to clearly show interference effects, it must overlap "exactly" and be at the right phase, or else any clear effects will be washed out.

Zz.

 

[cepheid]

I'm wondering about why "in some cases" and not others, though I guess your second sentence sort of addresses that. I'm not clear on what overlap "exactly" means. For instance, what happens when I stare at my reflection in a mirror. The light is clearly coming right back at me. What's going on there? Why are interference effects washed out?

 

[Danger]

I would think that it's pretty nearly impossible for a bunch of non-coherent light to randomly end up exactly out of phase with another batch. You probably have, once in a while, for a very brief time, a small fraction of the spectrum that interferes with itself, but how would you notice?

 

[ZapperZ]

I'm wondering about why "in some cases" and not others, though I guess your second sentence sort of addresses that. I'm not clear on what overlap "exactly" means. For instance, what happens when I stare at my reflection in a mirror. The light is clearly coming right back at me. What's going on there? Why are interference effects washed out?

But the light you are seeing of yourself in the mirror isn't from a coherent source. Besides, it didn't originate out of your eyes, so the path from,let's say your belly to your eyes do not overlap. So how are they going to "interfere"?

Zz.