What is a Photon? - Definition, Energy & Wavelength

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  • 0:01 Definition of a Photon
  • 1:22 Energy
  • 3:05 Wavlength
  • 3:42 Calculation of Photon Energy
  • 4:29 Lesson Summary
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Lesson Transcript
Instructor: Thomas Zesiger

Thomas has taught electronics and communications engineering, math, and physics and has a master's degree in electrical engineering.

In this lesson, we will learn the definition of a photon. We will also explain Planck's constant and its relationship to the photon energy and wavelength. The calculation of photon energy is also demonstrated.

Definition of a Photon

A photon is the quantum of electromagnetic radiation. The term quantum is the smallest elemental unit of a quantity, or the smallest discrete amount of something. Thus, one quantum of electromagnetic energy is called a photon. The plural of quantum is quanta.

The concept of photons and quanta comes from quantum mechanics and quantum theory. Quantum mechanics is a mathematical model that describes the behavior of particles on an atomic and subatomic scale. It demonstrates that matter and energy are quantized, or come in small discrete bundles, on the smallest scales imaginable. A photon propagates at the speed of light.

Illustration of wave-particle duality

A photon describes the particle properties of an electromagnetic wave instead of the overall wave itself. In other words, we can picture an electromagnetic wave as being made up of individual particles called photons. Both representations are correct and reciprocal views of electromagnetic waves. For example, light exhibits wave properties under conditions of refraction or interference. Particle properties are exhibited under conditions of emission or absorption of light.


The idea of quantum mechanics and photons originated from scientists' observations of the photoelectric effect. The photoelectric effect is where light striking a metal surface causes electrons to be ejected from the metal. Scientists were unable to explain this phenomenon, but eventually the explanation came from quantum theory.

What they found was that the energy in each quantum of light depends on the frequency of the light. In particular, the energy of a photon equals Planck's constant times the frequency of the radiation. Mathematically, this is given by the equation E = hf. Planck's constant is the fundamental constant of quantum theory that determines the scale of the small-scale world. Planck's constant = 6.63 * 10-34 joule-second (J-s). The total energy in an electromagnetic wave is the sum of the energies of each photon in the wave.

The energy of a photon is so small that we usually measure it in electronvolts (eV). One eV is the potential energy of each electron in a 1-volt battery. One eV is equal to 1.6 * 10-19 joules (J). Therefore, we need to convert Planck's constant to appropriate units, which are electronvolts/hertz (eV/Hz). In eV/Hz, Planck's constant is 4.136 * 10-15 eV/Hz.

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