45223wc@mtuxo.UUCP (w.cambre) (11/05/85)
What's a photon?
What happens when two photons hit each other?
Do photons lose energy when they bounce off things?
Do photons lose more energy when they bounce off black things
than when they bounce off white things?
How about mirrors?
If I have a hollow sphere with a mirror coating on the inside
and a light source in it, and say the light cannot get out of the
inside of the sphere, if I leave the light source on for a while,
will the sphere fill up with photons? Will it get full?
Will it get brighter and brighter in there? If I keep the light on
in there for 30 years then suddenly break open the sphere, will
I be blinded by all the light that explodes out of it?
What's the difference between photons in a red light and photons in a
blue light. I thought red and blue light are energy in a certain
wavelength. Does a photon have a wavelength?
Does anybody really know?
- Bill Cambre {ihnp4!}mtuxo!45223wc
P.S.: I just know my kid is going to ask me these questions some day.gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (11/08/85)
> What's a photon? A theoretical construct postulated to account for the particle-like aspects of light, specifically the photoelectric effect. Quantum field theorists think in terms of particles as the carriers of interaction; photons then are the particles for the electromagnetic interaction according to this theory. Most of the other questions have to be interpreted in the context of some theory or another; photons are not simple isolatable particles that can be studied in captivity. In particular, photons do not "bounce" off things. They may be absorbed by material particles and other photons emitted at the the same (or a later) time. The energy of the emitted photon could be different from that of the absorbed photon, depending on the state of motion of the material. > If I have a hollow sphere with a mirror coating on the inside > and a light source in it, and say the light cannot get out of the > inside of the sphere, if I leave the light source on for a while, > will the sphere fill up with photons? Will it get full? > Will it get brighter and brighter in there? If I keep the light on > in there for 30 years then suddenly break open the sphere, will > I be blinded by all the light that explodes out of it? Assuming you could prevent absorption of the light by the vessel, eventually an equilibrium would be reached between the field energy and the light source. Since the walls of the vessel will actually be absorbing and emitting radiation also, the system is much like the traditional "black body". If you pump enough energy into it, it will be glowing like a star (or other furnace) inside. The walls would also eventually glow on the outside, in practice. You can be blinded by peering into a furnace without proper eye protection.. > What's the difference between photons in a red light and photons in a > blue light. I thought red and blue light are energy in a certain > wavelength. Does a photon have a wavelength? The "wavelength" associated with a photon is determined by its energy: E = h * nu, where E is the energy, h is Planck's constant, and nu is the frequency. The frequency and wavelength are reciprocally related: lambda * nu = c, where lambda is the wavelength and c is the speed of light. Combine these and you get: E * lambda = h * c. Yes, spectral color is determined by wavelength. Note that a single photon of visible light contains an insignificant amount of energy compared to quantities encountered in daily life.
mwg@petrus.UUCP (Mark Garrett) (11/08/85)
++ > What's a photon? [etc...] > How about mirrors? > If I have a hollow sphere with a mirror coating on the inside > and a light source in it, and say the light cannot get out of the > inside of the sphere, if I leave the light source on for a while, > will the sphere fill up with photons? Will it get full? > Will it get brighter and brighter in there? If I keep the light on > in there for 30 years then suddenly break open the sphere, will > I be blinded by all the light that explodes out of it? Let's change the geometry slightly. Suppose you have two flat mirrors aimed at each other (or convex mirrors if you like; just something to contain the light and involve basically one dimension, for simplicity). Now put a light source in the middle. Light bulbs are too sloppy, spectrally speaking, so we use some distributed source; a material which emits light of a particular color (wavelength)* when activated by some energy source (other light, electricity etc). Now turn it on. The light intensity inside does build up to very high levels. However, it won't go on forever because all kinds of non-linear effects which were negligible for normal conditions have profound effects at high photon density. What does this mean? A "linear" effect would be a loss (like leakage or absorbance of imperfect mirrors) where for every 100 units of light you have, you loose 1 unit. A non-linear effect would have you loose 1 unit at low power levels, and a increasing fraction of the light as the total intensity increased. Therefore, as the intensity rises, you come to a point where things balance out. The sources of light equal the losses of light. This 'equilibrium' point yields extremely high light intensity, much greater than the brightness of the sun (measuring in light power per incident area, or energy contained per unit volume, not total power output, of course). To see this light, and use it, we make one of the mirrors slightly transmissive, so say, 1% of the light goes through, almost 99% is reflected and a small amount is absorbed. Then a thin beam of bright light comes out of the back of the mirror which has only 1% of the intensity inside the cavity (but is still very bright). This thing is called a laser (for Light Amplification by Stimulated Emmision of Radiation), and was invented by Townes and Schalow in 1958, and first constructed by Maiman in 1960. * There is a quantum mechanical effect involved that permits all the "wave packets" of light emmitted by the atoms of the material to be released "in phase" with the existing light, so there is no cancelation of one photon by another. > Does anybody really know? > - Bill Cambre {ihnp4!}mtuxo!45223wc Yes, we do! What you need is a fourth semester college physics text with lots of heuristic explanation (skip the equations). Look up lasers, the photo-electric effect, the Michelson-Morely experiment etc. Einstein was very good at explaining things for the layman, but I don't know if he wrote on light as much as he did on relativity. -Mark
ins_apmj@jhunix.UUCP (Patrick M Juola) (11/08/85)
In article <1092@mtuxo.UUCP> 45223wc@mtuxo.UUCP (w.cambre) writes: >What's a photon? A "piece" of light, or more specifically one quantum unit of a given frequency. >What happens when two photons hit each other? They just go through each other, like waves in water. >Do photons lose energy when they bounce off things? If they are absorbed and readmitted, yes. If the thing is moving away (making a Doppler shift), yes. Otherwise, no. >Do photons lose more energy when they bounce off black things >than when they bounce off white things? No. >How about mirrors? What about them? All a mirror does is reflect coherently, rather than scatteringthe light about. >If I have a hollow sphere with a mirror coating on the inside >and a light source in it, and say the light cannot get out of the ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ You can't GET a mirror like that; it violates conservation of energy. >inside of the sphere, if I leave the light source on for a while, >will the sphere fill up with photons? Will it get full? >Will it get brighter and brighter in there? If I keep the light on >in there for 30 years then suddenly break open the sphere, will >I be blinded by all the light that explodes out of it? What would happen would be that the mirror warms up to white-hot, so it shines light away as fast as you put it in. >What's the difference between photons in a red light and photons in a >blue light. I thought red and blue light are energy in a certain >wavelength. Does a photon have a wavelength? Yes, it does. Well, actually, the energy of a photon is related to the wavelength of the light it represents. This is related to the wave-particle duality of light; light is both a stream of discrete photons and a bunch of electromagnetic waves simultaneously; the waves have a wavelength, the photon merely has energy which is equal to the speed of light times Plank's constant over the wavelength. > >Does anybody really know? > No, all this is just a theory, but it's a fairly strong one. Does anyone really know that all mammals have hair? Nothing in science is ever definite. > - Bill Cambre {ihnp4!}mtuxo!45223wc Pat Juola Johns Hopkins Univ. Dept of Maths
mwg@petrus.UUCP (Mark Garrett) (11/08/85)
++ > ....The light intensity inside > does build up to very high levels. However, it won't go on forever because > all kinds of non-linear effects which were negligible for normal conditions > have profound effects at high photon density. Sorry, my mistake. You don't need non-linear effects (although they exist) to explain lasers. When the energy put into the system by the pump equals the losses due to the absorbtion and leakage (intentional and not), then the equilibrium light intensity is reached. -Mark
peter@graffiti.UUCP (Peter da Silva) (11/09/85)
> What's a photon? A packet of waves is as good a description as any. > What happens when two photons hit each other? They pass through each other. If the energy is high enough there may be particle-pair creation at this point, but that's unlikely with the photon- energies you're likely to meet. > Do photons lose energy when they bounce off things? No, though some observers may see a net loss or gain when the photon bounces off a moving object, or one with a relatively low mass. > Do photons lose more energy when they bounce off black things > than when they bounce off white things? No. It's just that more photons are absorbed by and fewer photons are reflected from black objects. > How about mirrors? See above. > If I have a hollow sphere with a mirror coating on the inside > and a light source in it, and say the light cannot get out of the > inside of the sphere, if I leave the light source on for a while, > will the sphere fill up with photons? Will it get full? Only with perfect mirrors. Since you can't build a perfect mirror outside a science fiction story the point is moot, but even if you could, eventually it'd stop filling up: The energy density on the sphere will increase. Some photons will escape due to quantum effects, and eventually this tunneling will reach the point where the same amount of energy escapes as is provided by the light. I guess you could call this "full". > Will it get brighter and brighter in there? If I keep the light on > in there for 30 years then suddenly break open the sphere, will > I be blinded by all the light that explodes out of it? Probably. I think it likely the sphere would explode unless you used Larry Niven's "stasis feilds" or E.E. Smith's "Zone of Force". > What's the difference between photons in a red light and photons in a > blue light. I thought red and blue light are energy in a certain > wavelength. Does a photon have a wavelength? Yes. See answer to first question. As an excersize calculate the wavelength of the monkey in the monkey-weight experiment. Calculate the probability of the monkey turning into a potted plant and a very surprised sperm whale. > Does anybody really know? Yes, I think. -- Name: Peter da Silva Graphic: `-_-' UUCP: ...!shell!{graffiti,baylor}!peter IAEF: ...!kitty!baylor!peter
sukenick@ccnysci.UUCP (11/14/85)
In article <petrus.679> mwg@petrus.UUCP writes: >++ >> What's a photon? [etc...] > >> How about mirrors? >> If I have a hollow sphere with a mirror coating on the inside >> and a light source in it, and say the light cannot get out of the >> inside of the sphere, if I leave the light source on for a while, >> will the sphere fill up with photons? Will it get full? >> Will it get brighter and brighter in there? If I keep the light on >> in there for 30 years then suddenly break open the sphere, will >> I be blinded by all the light that explodes out of it? Photons are particles with integer (zero) spin, so you can fit as many as you like in a container of any size without filling it up. (according to what I remember from my Quant course taken a few years ago) Under certain conditions, the intensity will increase but only to a point (see stuff further down) > [ stuff about lasers,etc.] > You don't need non-linear effects (although they exist) >to explain lasers. When the energy put into the system by the pump equals >the losses due to the absorbtion and leakage (intentional and not), then >the equilibrium light intensity is reached. > >-Mark Lasing(sp?) takes very special conditions to occur (the material the light is propagated through, distances between mirrors, etc.) Mark answered the question but then presented a special case for special conditions. Let me present a related question: Take a room of arbitrary size, dimensions and atmosphere and decorate walls, floors and ceiling with mirrors (The good ones, not the cheap tile mirrors:-)) Place a light source inside, any that you choose (light bulb, flash tube, mercury lamp). What will intensity of light in that room be, assuming the lamp and room corners absorb none of the light? Will the intensity of light be a function of time? If a hole is drilled from the outside, will laser light shoot out? Laser light will not be the result, under most conditions. The total intensity *might* increase to some value (and then stop increasing), depending upon the intensity of the light source and losses in the mirrors (NO mirror can reflect 100% - some of the incident photons will be absorbed and converted to heat or aborbed and re-emitted at lower frequency or just transmitted through; and as photon intensity increases, these effects increase.) If the photon flux from the source is greater than the absorption & losses from the reflectors, then the total amount of photons should increase, but will stop increasing at a point due to increased losses at higher intensities. We can calculate the gain: Number_photons = Number_photons - efficiency * Number_photons ^ ^ ^^^^^^^^ after a reflection before reflection Loss Where efficiency relates to the reflectivity of the mirror. Iterate for # of reflections (speed of light / average distance in room) eg: assume a 99.9% reflective spherical room 1000 meters diameter, source that outputs 10**20 photons in a flash at each second. Speed of light is 10**8 meters/sec, so in 1 second, the light gets reflected 10**8/1000 times/sec or 10**5. Efficiency = (100-99.9)/100; # of reflections = 10**5 and you get less than 1 photon at the end of a second (actually 10**-34) Increase the mirror efficiency to 99.99% and there will be some photons left over, so the overall amount of photons in the room will increase. However, more photons means higher intensity, so eventually mirror efficency will decrease and losses will increase. (Ok, Ok, there is probably a simplier method to find the total # of photons, than iterating, anyone out there know the equation?) -GDS- cucard!ccnysci!sukenick
mej@hlwpc.UUCP (Michael Jacobs) (11/19/85)
Several people have responded to this question saying that since there is no such thing as a perfect mirror, the light inside the mirrored chamber would increase only to a finite limit. This doesn't make sense to me; if you are constantly adding light (photons) and only a small fraction can go away (presumable as heat) then it seems to me the number of photons (and intensity of light) should continue to increase unbounded. Am I missing something here? By the way, a related question is, what would happen if you fired a strobe once in the chamber? I think the answer is that the light would echoe around the room for awhile and gradually die out as the mirrors heated up. :.
mwg@petrus.UUCP (Mark Garrett) (11/20/85)
++ > Several people have responded to this question saying > that since there is no such thing as a perfect mirror, > the light inside the mirrored chamber would increase > only to a finite limit. > This doesn't make sense to me; if you are constantly adding > light (photons) and only a small fraction can go away (presumable > as heat) then it seems to me the number of photons (and > intensity of light) should continue to increase unbounded. > > Am I missing something here? You're thinking in terms of losing a fraction of the incoming flow whereas the actual loss is a fraction of the total intensity of light. Here's a simple analogy with water: A tank is fed with a flow of water at the rate of 10 gal/sec. The tank leaks at a rate of 1% of its contents per second (not 1% of the influx). How much water accumulates? Well, when there's 1000 gallons, then the input equals the output. Of course, with light, the photons aren't sitting still, and the leakage really is a fraction of the photons *hitting the walls* (or interior material if you consider absorbtion). But ALL the light is constantly hitting the walls, not just the newly added light. Someone treated this in another posting as a series of strobe flashes. This is useful since it's easy to work with discrete events. To get a real situation, just take the limit as the summation approaches an integral. -Mark Garrett
crickman@umn-cs.UUCP (Robin Crickman) (11/23/85)
----- > Let me present a related question: > > Take a room of arbitrary size, dimensions and atmosphere and decorate walls, > floors and ceiling with mirrors (The good ones, not the cheap tile mirrors:-)) > Place a light source inside, any that you choose (light bulb, flash tube, > mercury lamp). What will intensity of light in that room be, > assuming the lamp and room corners absorb none of the light? The source can't emit if it can't absorb. Photons exempt from the exclusion principle. This means that any number can occupy the same space: any hole with photons coming out can have photons going in at the same rate. > Will the intensity of light be a function of time? If the mirrors are perfect, the light intensity will increase until it is the same throughout the room as at the surface of the source. At this point, the source will be emitting and absorbing at the same rate: equilibrium. > If a hole is drilled from the outside, will laser light shoot out? Not likely, unless you used just the right sort of source (ie, the right sort of gas discharge tube) and the geometry of the mirrors was just right. John Hasler, guest of ...ihnp4!stolaf!umn-cs!crickman
peter@graffiti.UUCP (Peter da Silva) (11/23/85)
> This doesn't make sense to me; if you are constantly adding > light (photons) and only a small fraction can go away (presumable > as heat) then it seems to me the number of photons (and > intensity of light) should continue to increase unbounded. > > Am I missing something here? Yes. As the intensity of light goes up so will the amount of heating, until eventually the loss to radiation/conduction/whatever will reach the same level as the gain from the light source. Of courtse if you had a perfect insulator around the mirrors... :-> > By the way, a related question is, what would happen if > you fired a strobe once in the chamber? I think the answer > is that the light would echoe around the room for awhile > and gradually die out as the mirrors heated up. Yes. Although if you are careful in your choiuce of materials and adjust the shape of the chamber properly what you'll have is a laser. > :. *** REPLACE THIS LINE WITH YOUR MESSAGE *** -- Name: Peter da Silva Graphic: `-_-' UUCP: ...!shell!{graffiti,baylor}!peter IAEF: ...!kitty!baylor!peter