Compton scattering is considered stronger evidence for photons because it demonstrates that light transfers both energy and momentum in a particle-like collision. The photoelectric effect clearly supports quantized energy transfer, but its evidence for photon momentum is less direct.
In the photoelectric effect, an electron absorbs energy from incident electromagnetic radiation. Einstein’s photon model gives
where is the maximum photoelectron kinetic energy, (hf) is the photon energy, and is the metal’s work function. The threshold frequency and immediate emission support energy arriving in discrete packets.
However, the photoelectric effect mainly tests the relation . It does not directly reveal the photon's momentum or track a collision between two particles.
In Compton scattering, an incident X-ray photon collides with an electron. The photon leaves with a longer wavelength, while the electron recoils. Treating the interaction as a relativistic two-particle collision and applying conservation of energy and momentum gives
The measured wavelength shift depends on the scattering angle , exactly as predicted when a photon has momentum .
| Evidence | Photoelectric effect | Compton scattering |
|---|---|---|
| Photon property demonstrated | Quantized energy, | Energy and momentum, |
| Interaction | Photon absorption | Photon-electron scattering |
| Key observation | Threshold frequency and electron kinetic energy | Angle-dependent wavelength shift and electron recoil |
The common misconception is that the photoelectric effect does not support photons. It does strongly support them; Compton scattering is simply more direct collision evidence because both conservation laws must work simultaneously.
This is HL-only E.2 Quantum physics. For an Explain question, link electron recoil and increased photon wavelength to conservation of energy and momentum, then state that the measured Compton shift supports photon momentum more directly than the photoelectric effect does.
