In the photoelectric effect, emission occurs only if photon energy hf is at least equal to the work function φ. If hf < φ, what happens?

• A. Electrons are emitted only if the light is intense enough
• B. No electrons are emitted
• C. Electrons are emitted with some kinetic energy
• D. All photons cause emission

Answer: (B)

The key idea is that emission requires enough energy from each photon to overcome the work function of the surface. Each photon brings energy hf, and to liberate an electron you need at least φ. If hf is less than φ, there isn’t enough energy in any photon to break free the electron, so no electrons are emitted—even if the light is very bright. The emission of electrons only becomes possible when hf ≥ φ, in which case any emitted electron would have kinetic energy hf − φ. Increasing intensity only means more photons and thus more electrons when hf ≥ φ, but cannot compensate for hf < φ.