The photoelectric effect: Suppose incident photons have energy E_ph = h f and work function φ. If E_ph > φ, electrons are ejected. If E_ph = φ + 1.2 eV, what is the kinetic energy of emitted electrons in eV?

• A. 0 eV
• B. 2.4 eV
• C. -1.2 eV
• D. 1.2 eV

Answer: (D)

In the photoelectric effect, the energy of each incident photon must cover the work function φ to release an electron, and any extra energy becomes the electron’s kinetic energy: KE = E_ph − φ.

Here E_ph = φ + 1.2 eV, so KE = (φ + 1.2) − φ = 1.2 eV. Thus emitted electrons have 1.2 eV of kinetic energy. If the photon energy were exactly φ, KE would be zero; if it were less than φ, no electrons would be ejected. The other numbers would require different excess energy or are not physical for this scenario.