In the photoelectric effect, what is the kinetic energy of the emitted electron when hf > φ?

• A. hf − φ
• B. φ − hf
• C. hf
• D. φ

Answer: (A)

Photons bring energy hf to the surface. To eject an electron you must spend an amount φ to overcome the binding (the work function). When hf is greater than φ, the extra energy hf − φ shows up as the electron’s kinetic energy. So the emitted electron’s kinetic energy is hf − φ. If hf = φ, the electron would have zero kinetic energy, and if hf < φ no emission occurs.