It is assumed that light does not propagate through a different medium as such but by interacting with electrons of atoms it reaches. So photons crash into electrons of the medium's atoms, elevate their energy level and when falling back to their original energy level, they themselves create photons of the wavelength h/E. The question is: what about direction? Let's say the point source is a laser whose photons pass through a glass, why does it not scatter everywhere as the electrons falling back emit photons in 360° x 180° direction?
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$\begingroup$ Because whatever you look along the kept direction is not the result of scattering. $\endgroup$– AlchimistaCommented Feb 6, 2019 at 11:56
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Glass typically has a very high bandgap due to its low refractive index so photons of visual wavelengths typically don't have enough energy to cause the excitation of electrons in glass since the photons would require an energy $\geq E_g$. Thus, the photons are simply transmitted through the glass since they don't have enough energy to interact with the electrons, and the direction and angle can be described with the refractive index.
Beside this, one can also discuss the topic of scattering, which can be quite complex if one wants it to be. Shortly put, the roughness of a surface quantifies the angle at which we'll observe scattered photons. Worst case would be Lambertian scattering, where it is scattered in all directions in a half-sphere and best case would be no scattering but only transmission.
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$\begingroup$ In which medium does happen what I described, ie photons in 360° x 180° direction? $\endgroup$ Commented Feb 6, 2019 at 19:42
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$\begingroup$ @Zurechtweiser I'd guess that certain gas clouds could exhibit such behaviour, with them having a mild optical thickness. You, however, ask about two different processes. I the beginning of your question you're talking about spontaneous emission, and in the end you're talking about scattering. The phenomena i guess you're talking about is possible, but i'd say that it would in that case be 360x360 degrees. You're essentially talking about spontaneous emission, and YES, it is radiating isotropically (in all directions), no matter what excited them from the beginning. $\endgroup$ Commented Feb 7, 2019 at 7:40
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$\begingroup$ @Zurechtweiser What COULD happen however with a relatively long lifetime for the excited electrons could be that later coming photons in the laser-beam could very well stimulate the excited electrons, producing light at the same phase and direction. In this case the medium would attenuate the power in the laser-beam and the rest would be emitted isotropically in the medium, which would be the losses. Now assuming that the medium doesn't have a radiation efficiency at 100%, we'd experience some loss due to heat as wel $\endgroup$ Commented Feb 7, 2019 at 7:42
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$\begingroup$ Excellent answer. See also physics.stackexchange.com/questions/7437/… $\endgroup$– vy32Commented Nov 10, 2019 at 15:51