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

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Discussion Overview

The discussion revolves around the phenomenon of light reflection and interference, specifically why incident light does not interfere with reflected light in a mirror. Participants explore the conditions under which interference can occur, the nature of coherent versus incoherent light sources, and the implications for everyday observations, such as seeing one's reflection.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions why reflected light does not interfere with incident light, suggesting that in some cases, such as in electromagnetic cavities, interference does occur.
  • Another participant notes that for interference effects to be observable with a laser source, the light must overlap "exactly" and be in the right phase, or else the effects will be diminished.
  • A participant expresses confusion about the term "overlap exactly" and wonders about the nature of light reflection when looking in a mirror, questioning why interference effects are not noticeable in that context.
  • One participant argues that it is nearly impossible for non-coherent light to randomly align out of phase with another batch, suggesting that any interference would be fleeting and difficult to detect.
  • Another participant emphasizes that the light seen in a mirror is not from a coherent source and points out that the paths of light from the body to the eyes do not overlap, which would prevent interference.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for light interference, with some suggesting that coherent light is required while others question the nature of light paths and coherence in everyday reflections. The discussion remains unresolved regarding the specifics of interference in various contexts.

Contextual Notes

Participants highlight the dependence on coherence and phase alignment for observable interference effects, as well as the complexity of light paths in practical scenarios like reflections in mirrors.

cepheid
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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?
 
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cepheid said:
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.
 
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?
 
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?
 
cepheid said:
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.
 

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