Homework problem 1.69 [ENDORSED]

[Teddy_Rosenbluth]

1.69 In a recent suspense film, two secret agents must penetrate a criminal’s stronghold monitored by a lithium photomultiplier cell that is continually bathed in light from a laser. If the beam of light is broken, an alarm sounds. The agents want to use a handheld laser to illuminate the cell while they pass in front
of it. They have two lasers, a high-intensity red ruby laser (694 nm) and a low-intensity violet GaN laser (405 nm), but they disagree on which one would be better. Determine
(a) which laser they should use and (b) the kinetic energy of the electrons emitted. The work function of lithium is 2.93 eV.

For this question: how do we know what the work function is?

[Alex Chen 1B]

They give you the work function of lithium in the problem, which is 2.93 eV. You can find the conversion from eV to J online and it's 1 J = 6.2415*10^18 eV. Since the work function is equal to the threshold energy necessary to eject an electron from the metal surface, you can use it to figure out the shortest wavelength needed to reach the threshold energy. The work function will probably always be given to us in future problems.