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 hand- held 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.
I understand that the violet laser should be used because it has a high enough frequency to eject electrons from lithium, but can someone explain how that and the photoelectric effect relates to the scenario? I'm having trouble applying it. More specifically, how does ejecting the electron prevent the beam of light from being broken (and thus the alarm sounding)? I don't think I would know to apply the photoelectric effect to this problem if it didn't mention the work function.