Electromagnetic radiation

Electromagnetic radiation is a collective name for a set of electromagnetic waves. The collection of electromagnetic waves of different wavelengths constitutes the electromagnetic spectrum. The part of the electromagnetic spectrum that we can see with our bare eyes is called visible light, or briefly light.

Since Albert Einstein's work of 1905, electromagnetic radiation is seen as a flux of photons (also known as light quanta) through space. This view is reminiscent of Isaac Newton's view, who saw light as a stream of corpuscles. This view was rejected in the early 19th century by Thomas Young and Augustin-Jean Fresnel, who considered light as waves—later in the 19th century seen as oscillations of the ether. An electromagnetic wave being characterized by its frequency ν, Einstein postulated photons to be packets of energy hν. These massless elementary particles move with the universal speed of light c. The symbol h is Planck’s constant. Photons of the same energy hν are indistinguishable, they are bosons. The number density of photons corresponds to the intensity of the electromagnetic (EM) radiation.

On Earth the most important source of EM radiation is the the Sun, and it is no coincidence that the maximum sensitivity of the eyes of most animals and humans is exactly at the most abundant part of the electromagnetic energy radiated by the Sun. This is the part of the EM spectrum that is the visible light. All life on Earth depends on the energy received from the Sun and by photosynthesis this energy is converted into plant life and atmospheric oxygen. The fossil fuels that modern society uses—gas, oil, and coal—are stored sources of energy received in the past from the Sun in the form of electromagnetic radiation.