Photoelectric Effect

1. Photoelectrons don’t care how bright the light is. They only respond if individual photons carry enough energy. Which condition must be true for photoelectrons to be ejected from a photosurface?

• 1. The beam must be continuous for at least a second

• 1. The photon frequency must exceed a certain minimum value

• 1. The light must come from the UV region

• 1. The photo surface must be positively charged

2. A student increases the brightness of a red laser but notices no photoelectrons are emitted from the photosurface. What’s the best explanation?

• 2. The laser is not focused properly

• 2. The frequency of red light is too low to overcome the work function

• 2. Red light produces too much heat

• 1. 2. Brighter light creates stronger electric fields that push electrons back

15. A photosurface with a work function of 3.5 eV is illuminated with a laser of energy 4.0 eV per photon. What would be the stopping potential to block photoelectrons?

• 15. 0.5 V

• 15. 1.0 V

• 15.  4.0 V

• 15. No photoemission occurs

14. What is the photon energy of ultraviolet light of wavelength 248 nm? (h = 6.63 × 10⁻³⁴ Js, c = 3.0 × 10⁸ m/s)

• 14. 5.0 eV

• 14. 4.5 eV

• 14. 6.2 eV

• 14. 8.1 eV

3. Two photons strike a photosurface: one from infrared light, and another from ultraviolet light. Which one is more likely to eject a photoelectron and why?

• 3. Infrared photon, because it has higher momentum

• 3. UV photon, because it has higher energy due to higher frequency

• 3.  Both equally, since energy depends on intensity

• 3. Neither, unless light is continuous

4. If you want to increase the kinetic energy of the photoelectrons, which strategy will actually work?

• 4. Use a more intense beam of the same frequency

• 4. Use a photon source with higher frequency

• 4.  Increase the size of the photosurface

• 4.  Decrease the resistance in the external circuit

5. Your teacher said, “photoelectrons jump out faster when the light is bluer, not necessarily brighter.” What physics concept supports this?

• 5. Energy of a photon increases with frequency

• 5. Electrons are attracted to blue photons

• 5. Blue light has greater intensity than red

• 5. Kinetic energy depends on wavelength squared

6. Suppose the light source is made ten times brighter but the photon frequency stays the same. What happens to the maximum kinetic energy of photoelectrons?

• 6. It becomes ten times higher

• 6.  It doubles

• 6. It stays the same

• 6. It decreases

7. Why does stopping potential change when the frequency of the incident photons is increased?

• 7. Higher frequency photons heat the surface, reducing friction

• 7. Photon energy increases, increasing photoelectron kinetic energy

• 7.  More photons mean higher charge density

• 7. Higher frequency reduces resistance in the circuit

8. If the frequency of light used on a photosurface is just below the threshold frequency, what should you expect?

• 8. Some photoelectrons will be emitted with less energy

• 8. No photoelectrons will be emitted, no matter how intense the light is

• 8.  Photoelectrons will be emitted with zero kinetic energy

• 8. Photoelectrons will be emitted only at higher temperatures