You shine intense red light and dim ultraviolet light on the same metal surface. The UV frequency is above the threshold; the red frequency is below it. Which produces ejected electrons?
AThe intense red light, because greater intensity delivers more total energy to the surface
BThe dim UV light, because its photon frequency exceeds the metal's threshold frequency
CBoth, because enough total light energy reaches the surface from both sources
DNeither, because both beams are too weak individually to eject electrons
Each photon interacts with a single electron in an all-or-nothing event. The red photon energy hf_red < φ, so no single red photon can free an electron — intensity (number of photons) is irrelevant below threshold. The UV photon energy hf_UV > φ, so even a single UV photon can eject an electron. Option A embodies the classical misconception that energy accumulates from many photons; quantum mechanics shows it does not.
Question 2 Multiple Choice
For a fixed frequency above the photoelectric threshold, doubling the light intensity will:
ADouble the maximum kinetic energy of the ejected electrons
BDouble the number of ejected electrons per second
CRaise the threshold frequency for the metal
DIncrease the stopping potential required to halt the electrons
Intensity equals photon flux — more photons per second, not more energetic photons. Doubling intensity doubles the number of photon-electron collisions and thus the electron ejection rate. Maximum kinetic energy K_max = hf − φ depends only on frequency and the work function, not on intensity. Stopping potential depends on K_max and therefore also doesn't change.
Question 3 True / False
According to the classical wave model, a very bright low-frequency light source should eventually eject electrons from a metal if given enough time.
TTrue
FFalse
Answer: True
This is what the classical model predicts — and this prediction is wrong. Classical wave theory treats light energy as continuously delivered to the surface, so sufficient time and intensity should always transfer enough energy to free an electron. The photoelectric effect's experimental result — that no electrons are emitted below the threshold frequency regardless of intensity or exposure time — directly refutes this prediction and requires the photon model.
Question 4 True / False
The maximum kinetic energy of photoelectrons emitted from a metal depends on the frequency of the incident light, not on its intensity.
TTrue
FFalse
Answer: True
K_max = hf − φ. The frequency f determines the energy of each photon; the work function φ is fixed for a given metal. Intensity controls how many photons arrive per second and therefore how many electrons are ejected, but each electron that escapes carries at most hf − φ of kinetic energy. Millikan's precise measurements confirmed this linear K_max versus f relationship and measured Planck's constant h.
Question 5 Short Answer
Why does the classical wave model of light fail to explain the photoelectric effect, and what aspect of the photon model resolves each of its failures?
Think about your answer, then reveal below.
Model answer: Classical waves predict: (1) electrons should accumulate energy over time, so any frequency should eventually eject electrons given enough time — but experiment shows an instantaneous threshold. (2) Higher intensity should produce higher-energy electrons — but K_max is independent of intensity. (3) There should be a delay before emission — but emission is nearly instantaneous. The photon model resolves all three: energy comes in quanta hf, so a single low-frequency photon cannot free an electron no matter how long you wait (resolves 1); intensity sets photon count not photon energy (resolves 2); a single photon-electron collision is instantaneous (resolves 3).
The failure is structural, not quantitative: the wave model is wrong in kind, not just in degree. Millikan tried for years to disprove Einstein's equation and instead confirmed the linear K_max vs. f relationship and measured h to five significant figures — the same h as in Planck's blackbody law, cementing the photon concept across phenomena.