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From: sukenick@ccnysci.UUCP
Newsgroups: net.sci
Subject: Re: Questions on PHOTONS
Message-ID: <169@ccnysci.UUCP>
Date: Thu, 14-Nov-85 13:57:03 EST
Article-I.D.: ccnysci.169
Posted: Thu Nov 14 13:57:03 1985
Date-Received: Sun, 17-Nov-85 07:19:57 EST
References: <1092@mtuxo.UUCP> <petrus.679>
Reply-To: sukenick@ccnysci.UUCP (George )
Organization: City College of New York
Lines: 75
Summary: 

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