Emission spectra and the photoelectric effect: evidence for photons

Two experiments show that light comes in packets of energy called photons, each with energy proportional to its frequency (E = hf). In the photoelectric effect, light shining on a metal knocks electrons out of it, but only if the light's frequency is above a certain threshold. Below that frequency, no electrons come out no matter how bright the light is; above it, brighter light ejects more electrons, while higher-frequency light ejects them with more energy. A wave model cannot explain this, but the photon model can: each photon gives its energy to one electron. Emission spectra point the same way. When an atom's electrons drop from higher to lower energy levels, they emit photons with only specific energies, so each element gives off its own pattern of bright lines at particular wavelengths, which can fall in the visible, ultraviolet, or infrared parts of the electromagnetic spectrum. Astronomers use these patterns to identify the elements in distant stars.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report