In the photoelectric effect, what condition must be met for electrons to be emitted?

• A. The photon energy hf must be at least the work function φ; hf ≥ φ
• B. The photon energy hf must be less than φ
• C. The photon energy hf must be zero
• D. The work function φ must be greater than hf

Answer: (A)

The essential idea is energy conservation at the surface: photons bring energy hf, and to release an electron you must supply at least the work needed to overcome the surface barrier, called the work function φ. So emission occurs only if the photon energy is at least φ. If hf equals φ, the electron would barely escape with zero kinetic energy; if hf is greater than φ, the extra energy (hf − φ) becomes the electron’s kinetic energy. If hf is less than φ, no electrons are emitted regardless of light intensity.