Re: [Phys-l] Is light pure energy?

[John Denker <jsd@av8n.com>]

J. Schnick asks: "Is light pure energy?"

On 07/03/2007 07:15 PM, Dan Crowe wrote:
> Not in my book.  Energy is one property of light.  Light also has
> momentum, frequency and polarization/helicity.

Agreed.

I might have added that light has observable electromagnetic fields
that other forms of energy might not have.  Also note that the
polarization/helicity is also connected to angular momentum,
which is often well worth paying attention to.

> > The phrase "converted totally to energy" bothers me. 

Me too.

> > I think I would
> > have said something to the effect that the matter is converted into
> > light that has the same amount of energy m_o c^2 that the matter had
> > prior to the conversion.  Is light pure energy?

I would have said _rest energy_ in the amount of m c^2.
The electron and positron had mass, and therefore had rest energy,
whereas the photon is massless (and restless).
  http://www.av8n.com/physics/mass.htm

I think we can agree that the electron and positron were _matter_,
whereas the photons are non-matter.  I don't know a simple word that
means non-matter;  lots of people (not just Feynman) say "energy"
when they mean "non-matter" ... but it is definitely an abuse of
the terminology.