Midterm Solution Q2B

[Hailey Kim 4G]

Q2B. A beam of light at constant intensity and increasing frequency is shown on a metal surface. A detector reads the number of electrons ejected from the sheet of metal. The graph below has a frequency of the incoming light on the X axis and the number of the ejected electrons on the Y axis. Draw the expected plot for the number of the electrons as a function of frequency. Write on the X axis the frequency at which electrons will start to be ejected.

Can someone please explain how to go about this problem and explain why the frequency at which the electrons will start to be ejected is 1.23x10^15 Hz?

[kevinchang_4I]

The electrons begin to be ejected at 1.23 x 10^15 Hz because at this point the energy of the incoming photon is enough to overcome the threshold energy of the gold surface. Remember:

Ephoton = Ethreshold + Eemitted electron

[ishaa Diwakar 4E]

Also, once the light passes the threshold energy, it can eject electrons. Since the energy of the light is directly correlated to its frequency, increasing the frequency will only cause the energy of the emitted electron to increase. Therefore, the graph you draw should show a horizontal line (at any point of # of electrons) after the calculated frequency.