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Physics · Glossary

What is The photon?

Also known as: photon · Planck's constant

Definition 34.1 High School Physics · Chapter 34 — The Quantum World: Photons and Energy Levels

Light of frequency ν\nu exchanges energy with matter only in whole grains called photons, each of energy

E=hν=hcλ,h=6.63×10−34 J s,E = h\nu = \frac{hc}{\lambda}, \qquad h = 6.63 \times 10^{-34}\,\mathrm{J}\,\mathrm{s},

where hh is Planck’s constant (Planck, 1900; Einstein, 1905) and λ\lambda the wavelength in vacuum. A beam’s intensity measures the number of photons per second; each grain’s energy is fixed by the frequency alone.

One scale for the electromagnetic family (): wavelength (top), energy per photon hc/ (blue), from radio grains of a microelectronvolt to the MeV gamma grains of — yet a 1.0\, mW pointer pours 3 × 1015 grains per second: everyday light flows as smoothly as poured sand.
One scale for the electromagnetic family (Chapter 22): wavelength (top), energy per photon hc/λhc/\lambda (blue), from radio grains of a microelectronvolt to the MeV\mathrm{MeV} gamma grains of Chapter 19 — yet a 1.0 mW1.0\,\mathrm{mW} pointer pours 3×10153 \times 10^{15} grains per second: everyday light flows as smoothly as poured sand.
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