Photoelectric effect describes what occurs when

• A. A low energy photon strikes an atom, expending energy to eject an electron; just one electron is ejected.
• B. A high-energy photon is absorbed by the nucleus.
• C. Neutron emission results.
• D. A photon interacts with electrons without ejecting them.

Answer: (A)

Photons can transfer their energy to a bound electron. If the photon has enough energy to overcome the electron’s binding energy (the work function), it can eject that electron from the atom. The ejected electron’s kinetic energy is the excess energy of the photon beyond the binding energy, given by hν minus φ. If the photon energy is below the binding threshold, no electron is ejected. Each absorbed photon can eject at most one electron, so the number of emitted electrons scales with the photon flux (light intensity). This differs from processes that involve the nucleus or neutron emission, or from interactions where the photon is scattered without ejecting an electron. The key point is the energy must be sufficient to overcome the binding energy for electron ejection.