22.1 Energy and momentum of a photon

Electromagnetic radiation shows wave behaviour such as interference and diffraction and particle behaviour such as quantised photon interactions.

Use the model that explains the observation; neither classical wave nor classical particle language alone covers every experiment.

A diffraction pattern supports wave behaviour, while the photoelectric effect requires discrete energy transfers.

Wave–particle duality is not a claim that light alternates between two physical substances during travel.

A photon is a discrete packet of electromagnetic energy with energy proportional to frequency and zero rest mass.

Photons travel at c in vacuum and are absorbed or emitted as whole quanta in interactions.

A higher-frequency ultraviolet photon carries more energy than a visible red photon.

A photon is not a tiny classical wave crest, and increasing intensity increases photon number at fixed frequency rather than each photon’s energy.

The energy of one photon is E=hf=hc/λ, where h is Planck’s constant, f frequency and λ wavelength.

Use frequency in hertz or wavelength in metres and check that higher frequency means higher photon energy.

Halving wavelength doubles photon energy in vacuum.

E=hf is energy per photon, not total beam energy; total energy also depends on how many photons arrive.

One electronvolt is 1 eV=1.60×10⁻¹⁹ J, the energy transfer when a charge of magnitude e moves through 1 V.

Use eV for particle-scale energies and convert to joules when applying SI equations or comparing macroscopic work.

A 5.0 eV electron has energy about 8.0×10⁻¹⁹ J.

An electronvolt is an energy unit, not a voltage or an electron’s mass.

For a photon, momentum magnitude p=E/c=h/λ. Photons carry momentum despite having zero rest mass.

Use photon energy from frequency or wavelength, then divide by c; momentum direction follows propagation direction.

Shorter-wavelength photons have greater momentum because p=h/λ.

Zero rest mass does not mean zero momentum or zero pressure when photons are absorbed or reflected.