Particle Nature of Light: the Photon

The Photon: Light as a Particle

The photoelectric effect forced physicists to accept a deeply strange idea: light, when it interacts with matter, does not behave like a continuous wave. Instead, it exchanges energy in discrete packets, or quanta, each carrying an energy $E = h\nu$. But is a quantum of light merely a unit of energy, or can it be associated with a particle?

Einstein took the next logical step. If a light quantum has a definite energy, it should also have a definite momentum. Using the relation between energy and momentum for a massless particle travelling at speed $c$, he arrived at a momentum $p = h\nu/c$. A particle with both a fixed energy and a fixed momentum is a very concrete object. This particle was later named the photon.

The particle nature of light was not just a theoretical speculation. In 1924, A.H. Compton performed experiments scattering X-rays off electrons. The results showed that the scattered X-rays changed wavelength in a way that could only be explained if the X-ray beam behaved as a stream of particles colliding with electrons — a direct confirmation of the photon picture.

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Properties of the Photon

The photon picture of electromagnetic radiation can be summarised through five key properties. Each one follows directly from the experimental evidence and the theoretical framework Einstein built.

(i) Radiation behaves as if it is made of particles called photons

When radiation interacts with matter — whether in the photoelectric effect, Compton scattering, or the absorption and emission of light by atoms — it does not behave like a smooth wave. It behaves as if it consists of discrete, localised packets of energy. These packets are the photons.

(ii) Each photon has energy $E = h\nu$, momentum $p = h\nu/c$, and speed $c$

This is the central quantitative statement. For a photon of frequency $\nu$ (or wavelength $\lambda = c/\nu$):

The photon always travels at the speed of light $c$ in vacuum. It has zero rest mass — if it had mass, it could not travel at $c$.

(iii) All photons of a given frequency have the same energy and momentum, independent of intensity

This is a crucial point that distinguishes the photon picture from classical wave theory. For light of a fixed frequency $\nu$ (or wavelength $\lambda$), every single photon carries exactly the same energy $h\nu$ and the same momentum $h/\lambda$. The intensity of the light beam does not change the energy per photon.

What does intensity mean in the photon picture? Increasing the intensity of light of a given wavelength simply means increasing the number of photons per second crossing a given area. Each photon still has the same energy $h\nu$. So a brighter beam has more photons, not more energetic photons.

(iv) Photons are electrically neutral and are not deflected by electric or magnetic fields

Since a photon carries no electric charge, it experiences no force in an electric field or a magnetic field. This is why light travels in straight lines through uniform fields — it is not bent or deflected. (Gravitational deflection of light, predicted by general relativity, is an entirely different effect and is negligible in most laboratory situations.)

(v) In a photon-particle collision, total energy and total momentum are conserved; the number of photons may not be …