I've been trying to wrap my head around what determines whether a ray of light is being reflected or refracted. A beam of light does both, as shown in this picture.
http://sc663dcag.weebly.com/uploads/2/4/1/1/24110261/8991488_orig.jpg
But a beam of light is made out of multiple photons or waves.
1) So what about each individual photon; does the photon split into two, and one half is reflected while the other one is refracted?
2) Or does some photons reflect and some refract, and if so what determines what happens to which photon?
3) Or is it that photons just bounce of in different directions, some are bouncing away from the material causing reflection and some are bouncing into the material and thus causing refraction?
Well actually photons don't really bounce, do they, like ping pong balls bouncing of a basket ball? Photons are absorbed by an electron, exciting it to move to a higher energy level and then releasing that same energy again when falling back to it's ground state. But is this re-emission happening in random directions, sometimes away from the material and sometimes into the material? But if so, wouldn't that cause very diffuse reflections and refractions if just all photons bounce off in all random directions. This isn't happening, so what makes sure that the angle of reflection is the same as the angle of incidence (also viable in the image above)?
If an electron is absorbing a photon while it is circling around its atom, and then releases it again at a later time, then the electron isn't in the same position as is were when absorbing the photon, so how can the electron "know" what the angle of incidence was so that it can release the photon in a new angle that becomes the same as the incidence angle was at the time of absorption . . . that makes no sense, does it?!
What am I missing here? 🙂
Best Answer
This is one of the places where wave particle dualism gets some people in trouble. Many are taught that it means that light can be a wave and a particle, and that phrasing can lead to some confusion. I find it more intuitive to just rip the bandaid off quickly and say light is neither a wave nor a particle. It is something which, in some situations, can be well modeled as a wave, and in some situations can be well modeled as a particle, but it is its own thing (which can be well modeled in all known cases using a more complicated concept, a "wavefunction").
You can think of photons getting randomly reflected or transmitted on the boundary, but the truth is that the billiard-ball photon model really isn't very effective at describing what happens at this boundary. This is one of the regions where wave mechanics models the effects very well, while particle models don't do so well. If you use wave mechanics, the idea of a wave getting partially reflected and partially transmitted isn't difficult to believe at all. In fact, it's pretty easy to prove.
Thinking in wave terms at these boundaries also gives correct answers in peculiar situations where the particle model simply falls on its face. Consider the interesting case of an "evanescent wave."
In this setup, the laser and prism are set up at the correct angles to cause "total internal reflection." This means that, by the simple models, 100% of the light should bounce off the side of the prism and into the detector. Indeed, if the prism is in the open air, we do see 100% reflection (well, within the error bars of absorption). However, bring an object close to the prisim (but not touching) and things change. You end up seeing effects from the object, even though 100% of the light was supposed to be reflected!
If you think of light like photons, this is hard to explain. If you look at it as a wave governed by Maxwell's equations, you see that you would violate the law of conservation of energy if there was a "pure" reflection. Instead it creates a reflection and an "evanescent wave" which is outside the prism, and its strength falls off exponentially, which is really hard to explain with particles!
Of course, these too are all simplifications. The real answer to your question is that the wavefunction of the light interacts with the electromagnetic fields of the atoms in the prisim, and the result of that interaction leads to reflection, refraction, diffusion, absorption, and eveansecent waves. However, naturally those equations are a bit harder to understand, so we use the older, simpler models from before quantum mechanics. We just have to be sure to use the one which is most applicable in any given situation, because none of them are quite right.