Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!lll-crg!ucdavis!ucbvax!decvax!bellcore!petrus!mwg From: mwg@petrus.UUCP (Mark Garrett) Newsgroups: net.sci Subject: Re: Questions on PHOTONS Message-ID: <679@petrus.UUCP> Date: Fri, 8-Nov-85 10:30:06 EST Article-I.D.: petrus.679 Posted: Fri Nov 8 10:30:06 1985 Date-Received: Wed, 13-Nov-85 04:12:11 EST References: <1092@mtuxo.UUCP> Organization: Bell Communications Research, Inc Lines: 55 ++ > 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