> Obviously, the approximations we make for the rest energy of slow moving
> particles is completely wrong; for light, which has no mass, the second
> term of the first equation drops off altogether, and we get E^2 = p^2*c^2,
> or E = p/c. The energy on a photon is given by h*f where f is the
> frequency, so we can get f = p/h/c. Using the definition of wavelength
> that x = c/f, we get x = h*c^2/p.
Augh! It was right up until here... E = pc = hf = hc/x and therefore x =
h/p, the de Broglie wavelength... Silly me.