Light mass

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From: Geoff Ruth (gruth@leadershiphigh.org)
Date: Sun Nov 24 2002 - 14:17:59 PST 

Message-Id: <a0501040bba06ff9b3009@[192.168.1.12]>
Date: Sun, 24 Nov 2002 14:17:59 -0800
From: Geoff Ruth <gruth@leadershiphigh.org>
Subject: Light mass

My AP Chem students asked me a question that is baffling me. Using de
Broglie's wavelength equation:

    wavelength = Planck's constant / (mass * velocity)

Then you should be able to find the mass of a photon of light.
Moreover, the mass should change depending on the wavelength of the
light. However, I thought that photons had no mass.

I'm confused about this contradiction. My only hunch is that it has
to do with the difference between rest mass and relativistic - energy
mass. But even if this guess is true, I don't know how to explain
this to students.

Help!
Geoff

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