Apply the photon model to calculate photon energy and explain wave-particle duality.

What this dot point is asking

You need to apply the photon model to calculate photon energy and explain what wave-particle duality means.

Quantisation of light energy

Classical physics treated light energy as continuous. The photon model says light energy comes in discrete quanta - photons. A beam of light is a stream of photons; a brighter beam has more photons per second, but each photon's energy is fixed by its frequency.

Planck's constant

Planck's constant $h$ is the fundamental link between a photon's wave property (frequency) and its particle property (energy). Its small size ($\sim 10^{-34}$ J s) is why quantum effects are invisible in everyday life but dominate at atomic scales.

The electronvolt

Because photon energies are tiny in joules, the electronvolt is often used: $1 \text{ eV} = 1.60\times10^{-19} \text{ J}$. Visible-light photons carry roughly 1.8 to 3.1 eV. This unit makes atomic energy levels and photon energies easy to compare.

Wave-particle duality

Light behaves as a wave in some experiments and as a particle in others:

Crucially, light is not "sometimes a wave and sometimes a particle" in a contradictory way; it is a single entity whose behaviour is captured by quantum theory, with the wave and particle pictures each applying to particular experiments. The same duality applies to matter: electrons, normally thought of as particles, produce interference patterns too (the de Broglie hypothesis).