Threshold Energy

[Danieljm1B]

How do you find the threshold energy in a photoelectric problem?

[Ethan McCarthy 1F]

The threshold energy may be given in the problem. For instance, in the example Dr. Lavelle used in lecture on Friday, "If 3.61x10^-19J is required to remove an electron with zero kinetic energy from a metal surface, what would be the longest wavelength of light to do this?". Here the note "with zero kinetic energy" tells us that the given value (3.61x10^-19 J) is the threshold energy. Since the question is asking for the longest wavelength this means the lowest energy light (no excess used/converted to kinetic energy), AKA when E_kinetic = 0.

[David Zhang 1B]

You would use the equation: total energy of photon= threshold energy + kinetic energy. You would need to find/be given the energy of the photon and the kinetic energy in order to determine the threshold energy.