What Is a Photon?
A photon is the fundamental particle of light and all other forms of electromagnetic radiation. It is the smallest possible packet — or quantum — of electromagnetic energy.
A photon is a massless, chargeless elementary particle that carries electromagnetic energy. It travels at the speed of light (c = 299,792,458 m/s) in vacuum and has energy proportional to its frequency: E = hf, where h is Planck's constant. Photons exhibit both wave-like and particle-like behaviour — a property called wave–particle duality.
Key Properties of a Photon
Zero rest mass
Photons have no mass at rest, which is why they always travel at the speed of light. They cannot be slowed down or stopped in vacuum.
Energy: E = hf
Energy depends on frequency. A gamma-ray photon carries billions of times more energy than a radio-wave photon.
Momentum: p = h/λ
Despite having no mass, photons carry momentum. This is why light exerts radiation pressure on surfaces.
Speed: c = 3 × 10⁸ m/s
In vacuum, all photons travel at exactly the speed of light regardless of their energy or the motion of the source.
Spin: 1
Photons are bosons with spin quantum number 1. This means they can be polarised and do not obey the Pauli exclusion principle.
No electric charge
Photons are electrically neutral. They mediate the electromagnetic force but do not carry charge themselves.
Where the Idea Came From
In 1900, Max Planck proposed that energy is emitted in discrete packets (quanta) to explain black-body radiation. In 1905, Albert Einstein extended this idea, arguing that light itself is quantised — composed of individual particles (later called photons). His explanation of the photoelectric effect using this concept won him the 1921 Nobel Prize in Physics.
The photoelectric effect showed that light below a certain frequency cannot eject electrons from a metal surface, no matter how intense it is. This could not be explained by classical wave theory but was perfectly predicted by the photon model: each photon must individually carry enough energy (E = hf) to overcome the work function of the metal.
Wave–Particle Duality
Photons behave as waves when they diffract, interfere, and polarise. They behave as particles when they are emitted, absorbed, or counted one at a time by detectors. The double-slit experiment demonstrates both: individual photons land as dots (particles), but over many events they build up an interference pattern (waves). This duality is not a contradiction — it is a fundamental feature of quantum mechanics.
💡 Key concept
A photon is neither a classical wave nor a classical particle. It is a quantum object described by quantum electrodynamics (QED), and it behaves differently depending on what you measure.
Photons in the Real World
- Vision: Your eyes detect individual photons. Rod cells can respond to as few as 5–7 photons arriving together.
- Solar energy: Photovoltaic cells absorb photons and convert their energy into electricity via the photoelectric effect.
- Lasers: Produce coherent streams of photons with the same frequency, phase, and direction through stimulated emission.
- Medical imaging: X-ray photons pass through soft tissue but are absorbed by bone, creating diagnostic images.
- Telecommunications: Fibre-optic cables transmit information as pulses of photons at near light speed.
- Quantum computing: Photons are used as qubits in photonic quantum computers and in quantum key distribution for secure communications.
The Electromagnetic Spectrum
All photons are identical in nature — they differ only in energy (and therefore frequency and wavelength). From lowest to highest energy:
- Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays
A red photon (λ ≈ 700 nm) has about 1.8 eV of energy. A gamma-ray photon from nuclear decay can carry millions of eV.
The Sun emits roughly 10⁴⁵ photons every second. About 10²¹ of those hit each square metre of Earth's surface per second on a clear day.
People Also Ask
Does a photon have mass?
A photon has zero rest mass but possesses relativistic momentum (p = E/c). It is affected by gravity not because of mass but because it follows curved spacetime (general relativity). This is why light bends around massive objects — gravitational lensing.
Who discovered the photon?
Albert Einstein proposed the photon concept in 1905 to explain the photoelectric effect. Max Planck laid the groundwork in 1900 with energy quantisation. The word "photon" was coined by American chemist Gilbert N. Lewis in 1926.
What is the energy of a photon?
E = hf, where h = 6.626 × 10⁻³⁴ J·s and f is frequency in Hz. Equivalently, E = hc/λ. A visible-light photon has roughly 1.6–3.3 eV of energy. A dental X-ray photon has about 70,000 eV.
Can photons be destroyed?
Photons can be absorbed — their energy transfers to matter (e.g., an electron jumps to a higher orbital). They can also be created when charged particles accelerate or atoms transition between energy levels. In pair production, a high-energy photon can convert into an electron-positron pair near a nucleus.
How fast does a photon travel?
In vacuum, exactly 299,792,458 m/s (the speed of light, c). In a medium like glass or water, photons interact with atoms and the effective speed of light is reduced (e.g., ~200,000 km/s in glass), but individual photon-atom interactions still occur at c.
References and further reading
- Griffiths, D. J. & Schroeter, D. F. Introduction to Quantum Mechanics, 3rd ed. Cambridge University Press, 2018.
- Sakurai, J. J. & Napolitano, J. Modern Quantum Mechanics, 3rd ed. Cambridge University Press, 2020.