jspenguin wrote:Interesting question: when a photon hits a mirror, does the original photon bounce or does it get absorbed, then a new photon is emitted? Is there, physically speaking, a difference?
Wow -- this is one of those times that the comic was great and the alt-text was cool, but the forum discussion made everything great. JSPenguin, great job with sparking a MASSIVE discussion with such a simple question....
I only recently figured out the neat fact that Randall references in the alt text. So, (obligatory) GOOMHR.
I'm a college senior finishing my B.S. in physics. My primary fields of research have been optics, astrophysics, and relativity, so I should be able to provide SOME decent answer to this.
atimholt wrote:Feynman's QED actually explains the reflection thing really well. It all has to do with the wave-like nature of light, and the reflecting surface not absorbing too much of it.
Most "sciencey" people are familiar with Einstein's two Postulates:
- Postulate 1: The laws of Nature, the ways in which things behave, are the same in all inertial systems regardles of their speeds.
- Postulate 2: The speed of light in a vaccum is completely independent of the motion of the source emitting it.
What is less well-known is that he had a third and fourth Postulate:
- Postulate 3: Feynman will always be right.
- Postulate 4: Whoever cites Feynman will automatically win.
So, yes. Feynman is right. However, in this case, citing Feynman gives an incomplete picture of the problem, because he was primarily discussing the issue based on semi-classical wave equations. The original question was "when a (singular) photon hits a mirror....", so we are dealing with individual photon behavior, not the wave nature of light. A beam of light is a wave with stream-of-particle characteristics; a photon of light is a particle with wave-like characteristics. Based on Randall's comic, it is the individual photon that we want to address (keeping in mind its wave-like properties).
The important thing to remember is that this is completely different from normal "absorption" that we would generally imagine happening. Spectral absorption takes place when a broad spectrum of light passes through a particular gas. Any photons whose wavelength gives them the specific energy of an electron excitation in that gas (for instance, one of the hydrogen excitations from n' = 2, given by the Balmer series, has an energy of 2.82 eV, which corresponds to a photon wavelength of 410 nm) is completely absorbed by one of the atoms in the gas, exciting that atom (this is why there an absorption line in the solar light spectrum at 410 nm). Note: this is one way we can determine precisely the degree of redshift in light from a distant star; it is not merely the overall spectrum that is redshifted, but the specific spectral line structures themselves.
When an individual photon strikes a mirror, however, a few things happen. A mirror is some metallic/reflective surface covered by a pane of glass. Since the refractive index of air is different from that of glass, the photon is slowed down as it enters the glass, and the wavelength is compressed so the energy of that photon stays constant. Assuming that the photon's path was not perfectly perpendicular to the mirror, this wavefront compression will translationally alter the path of the photon by some angle. Thus, the particle's direction changed because of a wave property interaction.
When the photon strikes the boundary between the glass and the reflective surface, the same kind of thing happens. As a wave, the photon is absorbed and re-radiated, so the particle itself bounces away from the surface. It is the same photon, but the wave that composes it has undergone a transformation that changed its direction.
So, the answer is this: the wave-like part of the photon is absorbed and re-emitted; the particle-like part of the photon simply bounces. At no point in time does the photon cease to exist. This is different from spectral absorption/emission, where the photon is completely absorbed and ceases to exist, then has its energy emitted as a new photon a few microseconds later.
A decrease in light intensity after reflection is not the result of anything that happens to the individual photon, but is rather a consequence of impurities in the mirror that cause a few photons to be completely absorbed (like spectral absorption) and not reflected at all. Usually, this absorption does not result in any subsequent emission; the energy is just spread out to the rest of the mirror as a tiny bit of heat.