Re: pinhole Light mass

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From: pauld@exploratorium.edu
Date: Mon Nov 25 2002 - 08:57:07 PST 

Message-Id: <200211251657.gAPGv0a00133@isaac.exploratorium.edu>
From: pauld@exploratorium.edu
Subject: Re: pinhole Light mass
Date: Mon, 25 Nov 2002 08:57:00 US/Pacific

 Hi Geoff

Photons have no rest mass, but they are never at rest either.

However they do have mass. Everything with energy has mass through the famous
equation

 E = mc^2 so m = E/c^2

and the energy of photons is dependent on their frequency E = hf

so m = hf/c^2 gives the mass of a photon.

Photons do fall under gravity (this has been observed)

And photons do gravitationally attract other photons in theory, I'm not sure this
has been observed yet.

Paul D

> 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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