So the speed of light is slower when traveling in glass or water. I’ve been told that the photons are being absorbed and re-emitted by the molecules of the medium, and that accounts for the reduced speed, but if that’s true, why does it continue on in a beam that closely resembles the original one?
Ah! The joys of quantum electrodynamics!
Although your explanation is a little oversimplified, the oversimplified reply is that the absorption/emission of the photons still conserves momentum.
Keep in mind that photons are massless.
The photon that enters the medium and the photon that exits the medium are not the same photon, but are identical.
OK, but what physical law says that the momentum would be conserved? I’ve never heard of a law about that. Is there one? Why would the emitted photon be thrown the same direction (mostly) that the absorbed one had been going?
And while we’re at it, I’ve got another light question.
If you shine a white light through a red filter, where did the energy for the other colored photons go? If they were used to excite electrons, why didn’t the electrons re-emit those same colors when they fell back? I’ve all ready been through the elementary-school answer of “green things absorb all the colors except green,” and I want something more. My science-geek father, my physics-major co-worker, and even my junior-college chemistry teacher have been unable to answer this.
Apparently, the electrical interactions of the bondings set up in certain molecules dictate the re-emission spectra of molecules. Chlorophyll always sends back green. So does the energy of the absorbed photons get absorbed by the entire molecule? Everyone knows that a bright light will heat up whole bodies of matter (witness McDonald’s), but how does that happen if it’s the electrons that would do the energy absorbing, and not the nucleus?
Law of Momentum Conservation. But does it apply to photons (objects lacking mass)? I’m pretty sure it does.
That’s just the thing… it’s not. Different materials that allow light to pass through also can cause light to exit at a different angle than the one with which it entered. Glass, water, and everything else like it has a “refractive index” that determines how much light bends. Air does as well, which is why you see a sunset even after the sun has gone below the horizon. Picture yourself standing near a pond. If you see a fish in the pond, the position you think the fish is at is not the actual position of the fish. You can notice this with an aquarium as well. Snell’s Law is n[sub]1[/sub]SinX[sub]1[/sub]=n[sub]2[/sub]SinX[sub]2[/sub]. This dictates the angle of emission, given the refractive indices for the two materials and the angle of entry.
I think your chlorophyll example is of a different nature, but it may correlate. I don’t have enough physics and bio/botany background to account for this. As for the white light and red filter question:
WAG= the light isn’t really pure red, but that’s the dominant color. Our eyes don’t pick up on the other interactions.
Also, concerning the nucleus/electron statement:
It’s not an issue of what the photon hits, the photon will simply excite the electrons regardless. The photon “dies” as the electrons are excited, but the electrons quickly fall back to their ground state, emitting an identical but new photon when they return (Conservation of Energy). But you’re taking issue with solid/opaque objects. They gather the energy more readily and don’t reflect light or let it pass through nearly as much. Therefor much energy is given off in the form of heat (this is half-WAG).
Real physics people can sweep in anytime now.
I told you it was oversimplified but I didn’t think you’d be questioning conservation of momentum. That’s one of the lynchpins of physics, if you’ve forgotten.
Ultimately, conservation of momentum is derived from the symmetry of space, but I’m not the one to explain that to you.
You may be confused because photons are massless and momentum is generally described as mass times velocity. But photons have momentum based on their frequency.
In answer to your second question the absorbed light is converted into heat. In one sense it is re-emitted as infrared light, but probably not how you were thinking. Don’t confuse the fact that light is sometimes absorbed by kicking electrons into new orbitals with the notion that this is the only way photons and atoms can interact. The photons can, in fact, merely “jostle” the atoms, i.e., heat them. Your red filter actually gets hotter from absorbing all those photons, although it might not be noticable without careful measurement.
The heat lights at McDonalds have a strong infrared component which is strongly absorbed by the food being kept and, especially at IR frequencies, they are pretty bright, so you notice the difference.
For the record, chlorophyll doesn’t “send back” green light – it absorbs red light and the resulting reflection, minus the red, is perceived as green. Here also, the energy is absorbed by the chlorophyll molecule. (In fact, that’s the whole point – the plant uses the energy captured by the chlorophyll to drive the necessary chemical reactions to sustain life.) And the leaf doesn’t have to absorb all the red light that hits it, just enough to alter the sprectrum that is reflected so that it appears green to our eyes.
OK, so chlorophyll doesn’t “send back” green; it’s absorbing the red. But what is happening to absorb the red? If the electron was excited to level X, why didn’t it fall down the same distance and re-emit an equivilent photon? Or does it fall in steps and emit 2 or 3 less energetic photons? How can it take (or blue for some other things) and release it as infrared, or even keep the energy as molecular vibrational energy (heat)?
I really don’t understand the comment about “jostling” molecules. Can part of a photon be absorbed? If it’s not influencing the electrons, then what? The nucleus is very deep inside the electron cloud, comparitively.
For that matter, if you heat something with electricity or a flame, it will start converting that energy to infrared photons and broadcast them. Does anyone appreciate how that happens?
Bump.
One last bump.
I thought photons did have mass. If they don’t, why does gravity affect them? If gravitation causes objects to attract each other directly by their mass and inversely by the square of the distance in between them, then the gravitational attraction between a massless photon and a black hole should be zero. Which it’s not, or the black hole wouldn’t have that name.
Technically photons have zero rest-mass. That means they would mass 0 when their velocity is 0. However, photons are never at rest, so this doesn’t come into play. Photons do have energy, and by E=mc^2, energy is equivalent to mass, so photons do have momentum and can be effected by gravity.
Then there was the fortune teller who had to avoid drinking diet beer before seances due to the INCREASED speed of Lite through a medium.
First off, heat is not infrared. Infrared EM is a wavelength (or wavelength region) that happens to excite the molecules of our cells. That is why it is perceived as warm and causes heating. But heat is a measure of the transfer of energy due to molecular motion.
“Jostling” molecules… I’m not a chemist or particle physicist, but here’s my take. The photon impacts the electrons of the atom/molecule. The energy is converted to kinetic energy of the electrons and thus the molecules themselves, rather than bumping them up in energy state. Bumping in energy state (orbital) is the process that allows them to fall and emit a new photon, but just transfering kinetic energy makes it hotter - increases temperature. Thus the loss of energy from that photon.
Regarding plants, the red light is the right wavelength to interact with the chlorophyll, and thus increases the energy to use in the chemical reaction. Thus the red photons are removed from the spectrum, while the rest is reemitted/passes through. Then you see green.
If you use a flame to heat an object, you are transferring kinetic energy to the atoms/molecules. Some of that energy might be converted to photons through the excitement of the electron energy state, thus the emission of photons and therefore light and IR.
About photons being massless but being affected by gravity - in a rest state, photons are massless. However, there is a kinetic energy effect due to relativity that causes a mass term. Regarding black holes deflecting photons, this is said to happen because mass distorts spacetime. Thus the photons are “following a geodesic” - following the straight line path through the curved space. Like drawing a straight line on a piece of paper, then curving the sheet of paper and watching the line curve. The line didn’t change what it was, but conformed to the new shape of the paper. That is a gross simplification of a geodesic. How this works on a quantum gravity level I think has not been determined yet. I don’t know that much about relativity, and find it all esoteric. It’s confusing to me, and I haven’t studied it in much detail.
Chlorophyll is a relative of the heme group which holds iron in your blood; in chlorophyll, IIRC, it holds a zinc ion. The zinc ion captures a photon, releasing an electron. The chlorophyll pigment then tries to stabilize itself by grabbing an electron from nearby pigments, until the electron “hole” thus created hits a protein molecule with which the chlorophyll is associated. This protein produces an electron by removing it from water, producing two H[sup]+[/sup] ions and one O[sup]2-[/sup] ions. The latter will combine with a like oxygen ion and be released as gaseous oxygen.
Grabbing this electron from water releases energy, which is used to start a chain reaction that eventually leads to the creation of ATP and NADPH, two major chemical energy sources in all living things. Thus the energy from the light is turned into an energy form usable by the plant.
Part of a photon can’t be absorbed. It can be fully absorbed, then re-emitted as several lower-energy photons as the electron drops through several energy levels, or it can have so much energy that it kicks an electron out completely, imparting its momentum to the electron. However, photons simply don’t get to the nucleus, except possibly at extremely high temperatures. Like in the core of a star.
LL
LL42, thanks. I think that answers a lot of this. My dad is really going to appreciate this especially. Of course, if anyone else has something informative to add, I’d welcome it.
Another way of looking at what happens is that there is a certain probability that the original photon will be absorbed, and a certain probability it won’t. The photon’s waveform will therefore extend past the atom, and interfere with the waveform of the emmitted photon. Near the surface of the object, this interference is constructive in one direction, and destructive in all others. If you do the math for which direction is constructive, you’ll get Snell’s law. Within the object, the constructive direction is the same as the original photon’s direction (original meaning the one before the interaction within the object, not the one from outside the object).
As for the different colors: there are different orbitals for electrons that are stable. If a photon allows an electron to go from one stable orbital to another, it wil be in a higher energy state, and will emmit another photon as it loses its energy. If the photon can’t push the electron into a stable orbital, the only way the electron can absorb the photon is if the orbitals are changed i.e. the whole atom, not just the electron, moves.