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From: gwyn@brl-tgr.ARPA (Doug Gwyn )
Newsgroups: net.sci
Subject: Re: Questions on PHOTONS
Message-ID: <2997@brl-tgr.ARPA>
Date: Thu, 7-Nov-85 20:21:28 EST
Article-I.D.: brl-tgr.2997
Posted: Thu Nov  7 20:21:28 1985
Date-Received: Sat, 9-Nov-85 05:24:43 EST
References: <1092@mtuxo.UUCP>
Organization: Ballistic Research Lab
Lines: 60

> 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.