Question

4.A photon of green light strikes an unknown metal and an electron is emitted. The voltage is set to 2 volts. The electron cannot make the journey to the second plate. What can be said about a similar experiment done with violet light? A. An electron may or may not be emitted in the second experiment. It cannot be determined. B. An electron will be emitted in the second experiment, but it cannot be determined whether it will reach the second plate. C. An electron will be emitted in the second experiment, and it will make it to the second plate. D. An electron will not be emitted in the second experiment.
3.Green light shines on an unknown piece of metal, and the lab instruments detect emitted electrons. Which of the following will be true if the light is changed to violet?
A. Violet light will cause electrons to be emitted at greater velocities than those removed by green light.
B. Violet light will cause electrons to be emitted at lower velocities than those removed by green light.
C. Violet light will cause electrons to be emitted only if the light is as intense as the green light.
D. It cannot be determined whether violet will cause electrons to be emitted or not.

Answer 1

4) The correct answer is:
B. An electron will be emitted in the second experiment, but it cannot be determined whether it will reach the second plate. 

In fact, violet light has higher frequency than green light. This means that photons of violet light carry more energy than photons of green light (remember that the energy of a photon is proportional to its frequency: $E=hf$), so when they hit the surface of the metal, more energy is transferred to the electrons. The electron was already emitted with green light, so it must be emitted also with violet light, given the more energy transferred. The electron will also have more kinetic energy when hit by violet light, however, we cannot determine if it will reach the second plate, since we don't know how much energy has been used to extract the electron from the metal (in fact, we don't know the work function of the metal, i.e. the energy needed to extract the electron)


3)  The correct answer is
A. Violet light will cause electrons to be emitted at greater velocities than those removed by green light.

In fact, violet light has higher frequency than green light. This means that photons of violet light carry more energy than photons of green light (remember that the energy of a photon is proportional to its frequency: $E=hf$), so when they hit the surface of the metal, more energy is transferred to the electrons. Therefore, the emitted electrons will have on average greater energy (and so, greater velocity) than those removed by green light.