# photons

Jonathan B. Marder marder at agri.huji.ac.il
Sun Jul 27 03:54:47 EST 1997

``` You can express energy of photons in many ways e.g. electron volts, kJ
per mole (of photons), and also by wavelength. In the latter case, the
most common unit is wave number expressed
*per centimetre*. e.g. If the wavelength of the light is 500 nanometres,
then the wave number is 20,000 per centimetre. Wave number is entirely
proportional to energy so it in fact *is* a measure of photon energy. So
for 500 nm light the following expressions of energy are equivalent:-
20,000 / cm
2.48 eV
240 kJ / mol
57  kcal / mol

So to answer your question, the photon has both energy and wavelength
and these are different descriptions of the same thing. This is Planck's
law.

Anne Heise wrote in article ...

>A friend asked me if a photon has a wavelength.  What is an entry-level
>yet accurate way to answer her question?
>
>What I suggested was that light can be viewed as a particle or a wave,
and
>that the wavelength determines the energy in the light.  Meanwhile
>light-as-photons is also energetic, but I think the energy of photons
is
>expressed some other way.
>
>Am I close? I'm a little embarrassed not to have an easy (and correct!)