HW Question 1.31

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Hannah Lee 2F
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Joined: Thu Jul 11, 2019 12:15 am

HW Question 1.31

Postby Hannah Lee 2F » Fri Oct 25, 2019 1:10 am

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.

SnehinRajkumar1L
Posts: 101
Joined: Thu Jul 11, 2019 12:15 am

Re: HW Question 1.31

Postby SnehinRajkumar1L » Fri Oct 25, 2019 11:03 am

If I understand this scenario correctly, I would assume that the beam of light is ejecting an electron from the lithium and it is detected by the sensor. This essentially is the photoelectric effect, as you have light pointing at a metal that is causing an electron to be ejected. You are dealing with light on metal, so it is the photoelectric effect.


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