Why light behaves as particles and electrons behave as waves
In the photoelectric effect, light falling on a metal surface ejects electrons, but only when its frequency is above a threshold value, no matter how bright the light is. Einstein explained this by treating light as packets of energy called photons, each carrying E = hν. A photon gives all its energy to one electron, so the maximum kinetic energy of the emitted electron is hν − φ, where φ is the work function of the metal. Increasing intensity adds more photons and therefore more electrons, but not faster ones.
Louis de Broglie proposed the reverse idea: moving particles also have a wavelength, λ = h/p, where p is momentum. For everyday objects this wavelength is far too small to notice, but for electrons it is comparable to the spacing of atoms in a crystal, which is why electron beams show diffraction. The Davisson–Germer experiment confirmed this. Together these results show that light and matter both have a dual wave-particle nature.
Why it matters
Solar cells, light meters and photodiodes in cameras all rely on the photoelectric effect, and electron microscopes use the short de Broglie wavelength of electrons to see details far smaller than light microscopes can. These ideas mark the start of quantum physics and lead directly into the study of atomic spectra, the Bohr model and modern electronics.
Key terms
- Photon
- A packet of electromagnetic energy whose energy equals Planck’s constant multiplied by its frequency.
- Work function
- The minimum energy needed to remove an electron from the surface of a particular metal.
- Threshold frequency
- The lowest frequency of light that can eject electrons from a given metal surface.
- De Broglie wavelength
- The wavelength associated with a moving particle, equal to Planck’s constant divided by its momentum.