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An interesting question popped into my head today regarding laser light. First, we know that all matter is made up of atoms, and that photons are considered to exhibit wave/particle duality. But if you shine a laser on a mirror, the light is reflected in a coherent way, i.e., the reflected laser beam remains concentrated and isn't spread out or noisy or diffuse. In other words, the angle of reflection equals the angle of incidence and it does not scatter.

The question is, why does it not scatter, given that a mirror is made up of atoms, photons should be hitting the atoms at different angles, and therefore scatter as they reflect from the mirror?

innisfree
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Archeus
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  • You're misusing the word "coherent." –  Feb 14 '19 at 02:57
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    "Coherent light is a beam of photons (almost like particles of light waves) that have the same frequency and are all at the same frequency. Only a beam of laser light will not spread and diffuse. In lasers, waves are identical and in phase, which produces a beam of coherent light." Note the "will not spread and diffuse" From a Google search for "coherent light" Seems OK to me. – Archeus Feb 14 '19 at 03:03
  • That quotation from an unlinked search result has several issues that (a) are too complex for a proper discussion in these comments and (b) are unrelated to the way that you have misused "coherent" in your question. – rob Feb 14 '19 at 03:09
  • So do you have an alternate, better word? – Archeus Feb 14 '19 at 03:11
  • @Dan "Specular" seems to be a key to the answer, and what you say seems to make sense. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm) https://en.wikipedia.org/wiki/Light

    https://www.webelements.com/silver/atom_sizes.html 1000 pm = 1 nanometre (nm, nanometer) Ag Atomic radius (calculated) 165 pm

     – Archeus Feb 14 '19 at 03:33
  • From another blog: Faulty perception. Laser light is highly concentrated, yes, thus when bouncing off a mirrored surface it appears to not have been diffused, but it has. In spite of it being highly concentrated (coherent) the photons contained in the beam still behave as photons. That means that when being reflected off a surface, each photon meets the surface at an angle, but the surface viewed under extreme magnification will show the the surface is microscopically rough, thus some photons will strike a surface slightly off angle and be deflected in a direction exiting the main beam. . . – Archeus Feb 14 '19 at 03:40
  • It is diffused. Because the beam is exceptionally narrow, the majority of the beam will remain intact and it is not possible to notice the scattering. Fortunately, for anyone asking this question, they finally succeeded in capturing the flight of a laser beam as it bounces off a pair of mirrors. Note how the beam has spread as it progresses. https://www.youtube.com/watch?v=Uq0H4-nvBB8&feature=youtu.be – Archeus Feb 14 '19 at 03:42
  • I have reviewed the question and tried to make it clearer. Perhaps the correct terminology (cf. coherent) could be explained in a answer. @user26 some of your comments here could probably easily form an answer - you are allowed to answer your own question. – innisfree Feb 14 '19 at 04:19
  • You're use of coherence is not incorrect provided you electric that it is transverse coherence, not longitudinal coherence. – KF Gauss Feb 14 '19 at 04:42

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