# Homework Help: Energy of an X-ray photon

1. Apr 22, 2010

### edwinem

1. The problem statement, all variables and given/known data
The X-ray spectrum of molybdenum has a particular characteristic spectral line of
wavelength 6.6 *10^–11 m. The ionisation energy of molybdenum is 20 keV.

Deduce that the energy of an X-ray photon of wavelength 6.6*10^–11 m is 19 keV.

2. Relevant equations
E=h*f
E=(h*c)/l
h=planks constant
c=speed of light
l=lambda(wavelength)

3. The attempt at a solution
Normally this is quite easy. You just plug in wavelength into the equation and it should give you the answer. However, the part that I do not understand is that in the markscheme the proper formula to use is E(ev)=(h*c)/(l*e). Were e is elementary charge. Now this does not make sense to me as (h*c)/(l*e)=V (Voltage). So could someone explain why I need to use elementary charge.

2. Apr 22, 2010

### ehild

Physicists often use eV as unit for energy instead of joule. 1 eV is the energy an electron gains when it is accelerated across 1 V potential difference. This energy is equal to 1( V ) * e (charge of electron) = 1.6 10-19 J.

ehild