When we illuminate a metal surface with monochromatic light, we observe a threshold frequency below which no photoelectrons are emitted. Which of the following is not a reason for this threshold?
1) electrons absorb all the energy from a single photon.
2) photon energy depends on frequency.
3) in monochromatic light, all photons have the same energy.
4) higher intensity light delivers more photons per unit time.
5) none of the above