Radiant energy is the potential energy stored in the propagating fields of electromagnetic radiation (such as light, X-rays, gamma rays, and thermal radiation) that can be described in terms of discrete packets of energy, called photons, or continuous electromagnetic energy waves.
The term “radiant energy” is most commonly used in the fields of radiometry, solar energy, and heating and lighting, but is sometimes used in other fields (such as telecommunications). In modern applications involving the transmission of power from one place to another, “radiant energy” is sometimes used to refer to the electromagnetic waves themselves, rather than their energy (a property of waves). The term “electroradiant energy” has also been used in the past.
The term “radiant energy” also applies to gravitational radiation. For example, the first gravitational waves ever observed were produced by a collision of a black hole that emitted about 5.3 × 1047 J of gravitational energy.
As energy, its unit in the International System of Units is the joule (J). The amount of radiant energy can be calculated by integrating the radiant flux (or power) with respect to time. To denote radiant energy, the symbol Qe is often used in the literature (where “e” stands for “energetic”, to avoid confusion with photometric quantities). In branches of physics other than radiometry, electromagnetic energy is denoted by E or W. The term is used specifically when electromagnetic radiation is emitted from a source into the surrounding environment. This radiation may be visible or invisible to the human eye.
There are two different views of how electromagnetic waves carry energy, and both are considered accurate because of the concept of wave-particle duality in quantum theory. Wave-particle duality states that electromagnetic waves act both as physical particles, photons, and as oscillating waves. Radiant energy can be seen as the energy stored in photons. It can also be seen as the energy carried in the oscillating magnetic and electric field produced by the wave motion of the electromagnetic wave.
Radiant energy is important for experiments and tasks related to open systems. In most sciences, an open system is any system that openly exchanges energy with its external environment. Solar energy involves an open system, since the sun’s electromagnetic rays are part of the environment while the solar energy collectors and the devices they power are part of the system. This energy is also used in photosynthesis as the waves are absorbed by chloroplasts in cells.