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A photon is the quantum of the electromagnetic field — a discrete packet carrying energy E = hf = hc/λ and momentum p = h/λ = E/c. Photons have zero rest mass and always travel at c. Despite behaving as particles in interactions (absorption, emission, scattering), they exhibit wave interference and diffraction. The photon picture unifies the results of blackbody radiation and the photoelectric effect and is the foundation for quantum electrodynamics.
By the early 1900s, two experiments had stubborn results that classical physics could not explain. Blackbody radiation — the glow emitted by hot objects — required energy to be emitted in discrete chunks. The photoelectric effect showed that light could only eject electrons from metals if its frequency exceeded a threshold, with intensity below that threshold making no difference at all. Einstein's 1905 insight unified both: light itself comes in discrete packets called photons, each carrying energy E = hf, where h is Planck's constant and f is the frequency.
The energy formula E = hf is the cornerstone of the photon model. Frequency — not intensity — determines how much energy each photon carries. A blue photon (high frequency) carries more energy than a red photon (low frequency). When you double the intensity of a laser, you double the number of photons arriving per second, but each photon still carries exactly the same energy as before. This distinction explains why only high-frequency light can eject electrons in the photoelectric effect: no amount of dim-but-frequent low-frequency photons compensates for each one individually lacking the energy to overcome the work function.
Photons also carry momentum, given by p = h/λ = E/c. Despite having zero rest mass, a photon has both energy and momentum — a feature you will need when studying Compton scattering, where photons collide with electrons like billiard balls and transfer measurable momentum. Photons always travel at c in vacuum; in a medium, the apparent speed is reduced because photons are repeatedly absorbed and re-emitted by atoms, but each individual photon travels at c between those interactions.
The strange and essential feature of photons is that they do not fit neatly into "wave" or "particle" categories. They interfere with themselves through double slits — a wave behavior — yet they deposit energy at discrete points on a detector — a particle behavior. This wave-particle duality is not a paradox to resolve but a feature of quantum reality to accept. The wavelength λ determines energy and momentum; the intensity determines the rate of photon arrival. Both descriptions are necessary.
The photon concept is the entry point into quantum electrodynamics (QED), the most precisely tested theory in physics. More immediately, it provides the tools to understand atomic emission spectra, laser operation, and photovoltaic cells — all phenomena that depend on energy being transferred in discrete quanta rather than continuously. The photon model is where classical electromagnetism ends and quantum mechanics begins.