A photon is the fundamental particle of light and all other forms of electromagnetic radiation. It is the smallest discrete amount, or quantum, of electromagnetic energy.

Introduced by Albert Einstein, the concept of the photon revolutionized physics by showing that light can behave as both a wave and a particle (a property known as wave-particle duality).

Photons have no rest mass and always travel at the speed of light in a vacuum. The energy of a single photon is determined entirely by its frequency.

The energy of a single photon is directly proportional to its frequency. This fundamental relationship is described by the Planck-Einstein relation:

E = hf

This is one of the most important equations in quantum mechanics. It directly links the wave property of light (frequency) to its particle property (energy).

Each component of the photon energy equation is a key physical quantity:

E: The Energy of a single photon, measured in Joules (J) or electron-volts (eV).

h: Planck's constant, a fundamental constant of the universe (≈ 6.626 x 10⁻³⁴ J·s). Its small value is why quantum effects are not noticeable on a macroscopic scale.

f: The frequency of the electromagnetic wave, measured in Hertz (Hz).

Since the frequency (f) and wavelength (λ) of light are related by the speed of light (c = fλ), we can express the photon energy in terms of its wavelength as well.

By substituting f = c/λ into the energy equation, we get:

E = hc / λ

This form of the equation is extremely useful because wavelength is often easier to measure experimentally than frequency.

It shows that photon energy is inversely proportional to its wavelength: shorter wavelengths correspond to higher energy photons.

The concept of photon energy is central to many technologies and natural processes.

LEDs (Light Emitting Diodes): An LED works by having electrons drop from a higher energy level to a lower one within a semiconductor. When an electron makes this drop, it releases a single photon with an energy (and thus a color) corresponding to the energy gap.

Lasers: A laser produces a coherent beam of light where all the photons have the exact same frequency and energy, giving the beam its unique properties.

Photosynthesis: In plants, chlorophyll molecules absorb photons of specific energies (mostly in the red and blue parts of the spectrum) to power the chemical reactions that convert carbon dioxide and water into glucose.

Medical Imaging (X-rays): X-ray machines produce very high-frequency, high-energy photons that can pass through soft tissues but are absorbed by denser materials like bone, allowing doctors to see inside the body.