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