Radiant energy

Radiant energy and electromagnetic radiation are two terms that refer to the same concept. Both refer to the propagation of energy through space in the form of waves. These waves appear as oscillating electric and magnetic fields,  the fundamental feature of electromagnetic radiation.

  • Electromagnetic radiation can be viewed as either electromagnetic waves or a stream of photons. These two perspectives are not mutually exclusive but rather complementary, as explained by the concept of wave-particle duality.
    • Wave Perspective: From this viewpoint, electromagnetic radiation is thought of as waves of oscillating electric and magnetic fields traversing through space. The energy of the radiation is distributed across the wave, with its intensity related to the amplitude and frequency of the wave.
    • Photon Perspective: Electromagnetic radiation can also be thought of as a succession of massless particles known as photons. Each photon carries a discrete quantum of energy, referred to as photon energy, directly related to the radiation’s frequency. This perspective acknowledges the particle-like behaviour of electromagnetic radiation.
    • The concept of wave-particle duality which has been developed in Quantum Field Theory, reconciles these contrasting viewpoints by claiming that electromagnetic radiation exhibits both wave-like and particle-like attributes. This duality has experimentally verified.
  • Radiant energy includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays. Each of these types of radiation is characterized by a distinct range of wavelengths and frequencies.
  • The quantity of radiant energy is typically measured in terms of radiant flux over time. Radiant flux represents the amount of radiant energy passing through a unit area per unit time. It is commonly expressed in units of watts per square meter (W/m²).
About light-waves & particles
  • Electromagnetic radiation and the electromagnetic energy it transports can be described in terms of waves.
    • Electromagnetic radiation (radiant energy) includes all wavelengths of light from radio waves to gamma rays.
  • Electromagnetic radiation can be described in terms of photons and their properties.
    • Energy: Photons have energy that depends on their frequency or wavelength. Higher-frequency photons have more energy than lower-frequency photons.
    • Number: The number of photons in a given electromagnetic radiation depends on its intensity or brightness. Higher-intensity radiation has more photons than lower-intensity radiation.
    • Direction: Photons travel in straight lines, but their direction can be changed by interacting with matter.
    • Polarization: Photons can be polarized, which means that their electric and magnetic fields oscillate in a particular direction.
    • Speed: Photons travel at the speed of light, which is approximately 299,792,458 meters per second in a vacuum.
About light, radiation, radiant energy & electromagnetic energy

There is a difference in meaning between the terms light, electromagnetic radiation, radiant energy and electromagnetic energy in physics.

Light
    • Light is best used to refer to the subset of electromagnetic radiation that is visible to the human eye, ranging from violet (shorter wavelengths) to red (longer wavelengths).
Electromagnetic radiation
    • Electromagnetic radiation refers to the transfer of all forms of electromagnetic radiation through space by electromagnetic waves and includes gamma rays, ultraviolet (UV), infrared (IR), X-rays, and radio waves, as well as visible light.
    • Electromagnetic radiation exhibits a wave-particle duality. This means it can behave like both a wave and a particle depending on the experiment or observation method.
Radiant energy
    • Radiant energy is most commonly used to refer to electromagnetic radiation carried by electromagnetic waves and photons.
    • Radiant energy can be measured using instruments such as photometers, which detect the intensity of light or other forms of electromagnetic radiation.
Electromagnetic energy
  • Electromagnetic energy is a more general term that refers to any form of energy that is carried by electromagnetic waves, including both radiant energy and other types of energy that are not radiant (e.g., static electric fields).
  • The type of energy associated with electromagnetic radiation is a measurable quantity in physics, and its measurement is essential for understanding and analyzing physical systems and processes.
  • The unit of measurement for electromagnetic energy in the International System of Units (SI) is the joule (J), which is defined as the amount of energy required to perform one joule of work
  • The electronvolt (eV) is another unit of energy commonly used in atomic and subatomic physics.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References