A photon is a tiny packet of light energy. Think of it as a “light‑ball” that travels at the speed of light, \$c = 3.0\times10^8\,\text{m/s}\$. Even though it has no mass, it carries both energy and momentum.
The energy of a photon is linked to its frequency, \$\nu\$, or wavelength, \$\lambda\$, by the famous equations:
In physics we often use the electronvolt (eV) as a convenient unit of energy. One eV is the energy gained by an electron when it moves through a potential difference of one volt:
\$1\,\text{eV} = 1.602\times10^{-19}\,\text{J}\$
Because the photon energy is usually very small compared to everyday energies, the eV scale makes calculations easier. For example, visible light photons have energies around 2 eV, while X‑ray photons can reach 1 keV (1000 eV).
Even though a photon has no rest mass, it still carries momentum. The relationship between energy and momentum for a photon is:
\$E = pc\$
Rearranging gives the photon momentum:
\$p = \frac{E}{c} = \frac{h}{\lambda}\$
In particle‑physics units we often write the momentum in eV/\$c\$:
\$p \;(\text{eV}/c) = \frac{E \;(\text{eV})}{c} \;\approx\; E \;(\text{eV})\$
Numerically, a 500 nm photon (green light) has:
Photon momentum is responsible for radiation pressure – the tiny push that sunlight exerts on a sail or a dust particle in space. It also plays a key role in processes like the photoelectric effect, where photons transfer enough momentum to free electrons from a metal surface.
| Wavelength (nm) | Energy (eV) | Momentum (eV/\$c\$) |
|---|---|---|
| 400 | 3.10 | 3.10 |
| 500 | 2.48 | 2.48 |
| 700 | 1.77 | 1.77 |
| 10 | 124.0 | 124.0 |
| 0.1 | 12400 | 12400 |