What it is
A photon is a quantum of electromagnetic radiation: the smallest amount of light (or radio waves, X-rays and so on) that matter can emit or absorb. A photon has energy E = hf, where f is the frequency of the radiation and h is Planck’s constant, 6.62607015 · 10⁻³⁴ J·s. With f = c/λ this becomes E = hc/λ; see photon energy and photon energy from wavelength.
Examples, with h = 6.62607015 · 10⁻³⁴ J·s and c = 299 792 458 m/s, the values in the table of physical constants:
- Green light, 500 nm. E = hc/λ ≈ 3.97 · 10⁻¹⁹ J ≈ 2.48 eV. A 1 W laser emitting this light sends about 1/(3.97 · 10⁻¹⁹) ≈ 2.5 · 10¹⁸ photons every second.
- Red light, 700 nm. E ≈ 2.84 · 10⁻¹⁹ J ≈ 1.77 eV.
- Violet light, 400 nm. E ≈ 4.97 · 10⁻¹⁹ J ≈ 3.10 eV, about 1.75 times the energy of a red photon, since 700/400 = 1.75.
- Radio wave, 100 MHz. E = hf ≈ 6.63 · 10⁻²⁶ J. Radio waves are made of photons too, but each photon is very small.
A photon has no mass but has momentum p = h/λ. At 500 nm it is about 1.33 · 10⁻²⁷ kg·m/s; see photon momentum.
What is not a photon:
- A beam of light. It is made of a very large number of photons.
- Sound. Sound is a mechanical wave in matter; there is no photon of sound.
- An electron. An electron has mass and charge; a photon has neither.
Why it matters
The photon explains what the wave model cannot. In the photoelectric effect electrons leave a metal only if each photon has enough energy, however bright the beam. In Compton scattering the photon behaves as a particle colliding with an electron. And spectra from atoms appear as lines because an atom emits a photon of a definite energy when it changes level; see the Bohr model.
The photon is also the simplest example of wave–particle duality: light shows interference like a wave and is absorbed in whole packets like a particle. How dangerous radiation is depends on photon energy; see the electromagnetic spectrum.
Common misconceptions
- “A photon is a tiny ball.” The model is a quantised energy in a field, not a small ball flying along a path. It cannot be followed like one.
- “Brighter light means more energetic photons.” Brighter light of the same colour means more photons, not more joules per photon. The energy per photon is set by the frequency.
- “Photons have no momentum because they have no mass.” They have p = h/λ, which is why light can push on a surface.
- “Red light that is bright enough frees electrons from any metal.” If the photon energy is below the metal’s work function no electrons are freed in ordinary light, however many photons arrive; see work functions.
Common questions
How do you calculate the energy of a photon?
With E = hf if you know the frequency, or E = hc/λ if you know the wavelength. For 500 nm it comes to about 3.97 · 10⁻¹⁹ J, or 2.48 eV.
Does a photon have mass?
No, its rest mass is zero. It still has energy and momentum.
Is light a wave or a particle?
Light shows wave properties in some experiments and particle properties in others. See wave–particle duality.