Re: pinhole Re: photon rest mass

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Apr 28 2004 - 18:08:29 PDT 

Message-Id: <l03110710bcb603102c80@[192.168.112.30]>
Date: Wed, 28 Apr 2004 18:08:29 -0700
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole Re: photon rest mass

Hi Charlie

I've been away teaching workshops in Italy, sorry it took so long to respond.

>I had a student ask a couple of questions in physics this week. We use the
>Hewitt text which is great. However, the discussion is just deep enough
>in the
>book to raise more questions to curious students.
>
>Thanks,
>
>Charlie Bissell
>Hillsdale High School
>San Mateo Union HS District
>
>1) How is it that photons have no rest mass? Has this lack of rest mass been
>measured?
Photons have energy and so they have mass via m = E/c^2
Photons always travel at C so the idea of the rest mass of the photon is a
strange one.
However for an electron the total energy is the sum of the kinetic energy
and the rest energy. The minimum total energy of an electron is its rest
mass energy.
The minimum energy for a photon is zero energy for very low frequency
photons so the photons have no minimum energy and so no rest mass.

>2) What determines which direction a photon will travel after it's been
>emitted from an electron? Like, what makes the surface reflect (send the
>photon
>back in the direction it came from) or be transparent (send it in the
>direction
>of travel)?

This is really tricky to do in words, harder than I can do as you are about
to see, however the next time you see me I can do it with drawings.

I'll see if I can do a web page on this.

Here's my poor attempt, I'll accept help.

The probability wave for a photon emitted by an atom is often emitted in
all directions. This probability wave determines the probability at all
points in space that another atom will have to detect the electron.
(Actually it is zero along an axial direction and maximum at the equator.
in an r = sin(phi)^2 spherical polar plot.

A photon probability wave is re-emitted in all directions by each atom,
with a phase delay in transparent materials. The sum of the re-emitted
waves from all atoms leads to refraction.

The phase delay in metals (mirrors) is greater than 180 degress, the sum of
the re-emitted waves from all the metal atoms creates reflection.

Now you see why Hewitt skipped this topic. ;-)

Paul D

>
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