When a laser is shined into a glass ball such as this video (link) or a bottle of water (link) the light is refracted through the whole transparent body. However, the intensity of the light scattered to the camera differs through the ball (or the bottle). For example, the green around the perimeter of the ball is significantly brighter than that in the centre. What is the mechanism behind this? Is is because even "monochromatic" lasers consists of different wavelengths and those are dispersed by the glass/ plastic jar?

Edit: changed "reflected" to "scattered"

  • $\begingroup$ The Stranger Theory of Light and Matter by Feynman. This 152 page book covers this in great detail. Turns out it pretty much a mystery. $\endgroup$ – Lambda Jun 7 at 2:15
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    $\begingroup$ The title seems misleading as the topic seems to be scattered rather than reflected light. $\endgroup$ – Duncan Harris Jun 7 at 3:02
  • $\begingroup$ Thanks, Duncan I fixed the terminology $\endgroup$ – user3549378 Jun 7 at 13:04

What you are seeing is the glass acting as a light guide by capturing light by total internal reflection. These guided modes circulate the object and either escape by scattering at a defect or leach out over time (eg cladding modes in an optical fibre escape when its bent). The scattering has a preference for coming out tangential to the sides, which is why you see more scattered light at the edges than the centre (surface is normal). At the edge where the surface is tilted there is also a greater contribution area wise to the scattering than when you look at the centre.

  • $\begingroup$ Can you explain why the scattering is greater in the tangential direction? "Preference" is not a scientifically useful term in this context. $\endgroup$ – S. McGrew Jun 7 at 5:18
  • $\begingroup$ If you consider the statistical ensemble of surface irregularities, dust particles and grease, the scattering cross section probability distribution will be strongly peaked directed to low angles of refraction; hence tangential to match the critical angle of total internal reflection about which a perturbation would act. $\endgroup$ – Paul Childs Jun 8 at 0:43
  • $\begingroup$ I believe that, but would hope to see an explanation of why the distribution is strongly peaked that way. $\endgroup$ – S. McGrew Jun 8 at 1:21
  • $\begingroup$ Only a small perturbation is required to make total internal refracted light escape. The assumption is that the manufactures of the glass ball and beaker did a good enough job that the irregularities are small, thus causing less of a perturbation and less of a deflection away from the tangent. $\endgroup$ – Paul Childs Jun 8 at 14:53
  • $\begingroup$ Maybe I'm being a bit picky, but it would help if you can illustrate why an arbitrary TIR ray that encounters a surface irregularity and is scattered, sends more light in the tangential direction than in other directions. Can you add an illustration to your answer? $\endgroup$ – S. McGrew Jun 9 at 3:12

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