Quantum Physics · revision map · A-Level 9702, Topic 22

Electromagnetic radiation is particulate: a photon is one quantum of energy, set only by its frequency.

• Higher frequency means a more energetic photon.
• The electronvolt is the handy energy unit: 1 eV = 1.6 × 10⁻¹⁹ J.
• One photon carries momentum p = E / c.

One photon frees one electron, instantly. Below the threshold frequency f₀ no electrons escape, whatever the intensity.

• Max KE depends on frequency only, not intensity.
• Brighter light (more intensity) gives more electrons, so a larger current.
• Emission needs f ≥ f₀; energy is not stored up over time.

Light and matter each show both natures. The photoelectric effect shows light as particles; interference and diffraction show it as waves.

• Electrons diffracted by a crystal prove matter has a wave nature.
• A faster particle has larger momentum, so a shorter wavelength.

An atom holds only discrete energy levels (energies are negative, with the ground state lowest).

• A jump down emits a photon; a jump up absorbs one.
• Fixed gaps give sharp emission and absorption lines.
• A bigger gap means a higher f and shorter λ.