Energy of a photon

[ryuichiro_3I]

We have two questions E=hv and E= work function + KE. If we are trying to find the energy of a photon why do we use E= work function +KE and not just E=hv?

[kaylavakilian3k]

It depends on the problem! If you are given the velocity of the ejected electron and the work function only, that is not enough to determine the energy of the photon using E = hv. In this case, you would have to solve for the energy of the photon by calculating the kinetic energy of the ejected electron and adding it to the work function. So I think both equations can be used depending on what information is given to you in the problem!

[ryuichiro_3I]

So energy of a photon when trying to find the wavelength should be work function + KE if the energy of the photon is not given?

[PranavKetharam]

So energy of a photon when trying to find the wavelength should be work function + KE if the energy of the photon is not given?

That is absolutely correct. Everything comes down to what is given and what is being asked. If the work function and kinetic energy is known then the work function+KE equation is ideal for calculating the energy of a photon. However, if only the work function is given and the kinetic energy is 0 but you need to find the longest wavelength, then ideally you should use E=hv and change the equation to look like wavelength=hc/work function. Then, if a follow-up question asks for the wavelength and a new ke is given then you should use work function (from the previous question) + KE = energy of a photon and you can use wavelength = hc/E to find the wavelength.