Photons and the photoelectric effect

At very small scales, light behaves as particles called photons as well as waves. The photoelectric effect and atomic spectra could only be explained this way.

Key facts

  • Photon energy E = hf = hc/λ (h = 6.63 × 10⁻³⁴ Js).
  • Photoelectric effect: hf = φ + KEmax.
  • Below the threshold frequency, no electrons are emitted however bright the light.
  • Electrons in atoms have fixed energy levels; jumping down emits a photon of exactly the energy difference.
  • 1 eV = 1.6 × 10⁻¹⁹ J.

Worked examples

Example 1

Find the energy of a photon of wavelength 500 nm.

  1. 6.63 × 10⁻³⁴ × 3 × 10⁸ ÷ 5 × 10⁻⁷
  2. ≈ 3.98 × 10⁻¹⁹ J (about 2.5 eV)

Answer: 3.98 × 10⁻¹⁹ J