Question

When I shine dim red light on a metal surface, some electrons are ejected. Which of the following would NOT significantly increase the number of electrons per second being ejected from the surface?
A. Shining bright red light on the surface.
B. Shining bright white light on the surface.
C. Shining dim (higher energy) purple light on the surface.
D. Shining bright (higher energy) green light on the surface.
E. All of the previous actions would increase the number of electrons per second being ejected from the surface.

Answer 1

Learn the concept of photoelectric effect. In photoelectric effect when a certain frequency of light is incident on a surface of an electron it escapes the metal surface using the energy of the photon. The kinetic energy of the electron depends on the energy of the photon and the work function of the metal surface.

Complete answer:
We know that when light is incident on a surface of an electron it escapes the metal surface using the energy of the photon. The escape of electrons depends on the work function of the metal surface. Now, the work function is the minimum energy of a photon needed to release an electron from the metal surface. Also, we know that the energy of the photon depends on the frequency of the electron so the work function of the metal surface depends on the frequency of the electron. The intensity of the light does not play any role to release an electron; it only controls the rate of electrons ejecting from the surface.

Now, in the question only red light releases electrons from the metal. Now, let’s check all option one by one:
Option A: It is true since the number of ejected electrons depends on the intensity of the light or the frequency of the photon for the work function of the metal surface.

Option B: It is also correct since the white light contains all the frequency of the lights so it does contain red light also.

Option C: It is incorrect since red light is needed to release any electron from the surface of the metal in the first place. So, shining dam (higher energy) purple light on the surface will not release any electrons.

Option D: It is also incorrect since red light is needed to release any electron from the surface of the metal in the first place so shining bright (higher energy) green light on the surface will not release any electrons.

Option E: Since as stated above (A) shining bright red light on the surface and (B) shining bright white light on the surface will only increase the number of electrons per second being ejected from the surface and (C) shining dim purple light on the surface or (D) shining bright green light on the surface will not have any effect on the surface, so this option is also incorrect.

Hence, option A and option B are the correct answer.

Note: The releasing of any electron only depends on the frequency of the photon.So, if some metal surface releases electrons for red light it will not release electrons for any other colour of light. The number of electrons strictly depends on the intensity of light subject to the same colour or frequency of light.