A Level Physics Practice Exam

What occurs to the emitted electrons when the intensity of the incident radiation is doubled while keeping the frequency constant?

1. The number of emitted electrons decreases

2. The kinetic energy of electrons doubles

3. The number of emitted electrons increases

4. The emitted electrons are unaffected

The correct answer is: The number of emitted electrons increases

When the intensity of the incident radiation is doubled while keeping the frequency constant, the number of emitted electrons increases. This is because intensity is related to the amount of energy delivered per unit area per unit time, which can be viewed as the number of photons striking a surface per second. In the context of the photoelectric effect, each photon incident on a material can release an electron if its energy (which depends on the frequency of the radiation) meets or exceeds the work function of the material. Doubling the intensity means that there are now twice as many photons hitting the surface per second compared to the original intensity. Since the frequency remains constant, the energy of each photon does not change, but the increased number of photons results in a greater number of interactions with the material, ultimately leading to more emitted electrons. This increase in the number of emitted electrons does not affect their kinetic energy since that parameter is determined by the frequency of the incident radiation rather than the intensity. Therefore, as a result of the increased intensity, more electrons are emitted from the surface of the material.