Electromagnetic radiation
lastmod 2018-08-17created 2017-03-022 edits
Electromagnetic waves, or electromagnetic radiation (EMR), is energy radiated by certain electromagnetic processes. Visible light is also an electromagnetic wave, while electromagnetic waves such as radio waves, infrared, ultraviolet, and X-rays are invisible to our eyes.
In classical mechanics, electromagnetic radiation consists of electromagnetic waves made up of synchronized oscillating electric and magnetic fields. They propagate at the speed of light in a vacuum. The oscillations of the two fields are perpendicular to each other and to the direction of propagation, forming a transverse wave. Ordered by frequency, from high to low or low to high, electromagnetic waves form the electromagnetic spectrum, which includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
Electromagnetic waves are produced when charged particles are accelerated, and these waves can subsequently interact with other charged particles. In this process, electromagnetic waves can carry energy, momentum, and angular momentum. The quanta of electromagnetic waves are photons. Photons have no mass but are affected by gravity. Because electromagnetic radiation moves rapidly away from the particles (or currents) that produced it, it can be described as the set of electromagnetic waves that propagate freely through space without continuing interaction with their source. For this reason, electromagnetic radiation is also called the far field. Conversely, the near field refers to the electromagnetic waves around the particles or currents that produced them. Static electricity and electromagnetic induction are examples.
In quantum mechanics, electromagnetic waves are made up of photons, the elementary particles involved in all electromagnetic interactions. Quantum effects, such as black-body radiation and quantum jumps to lower energy levels in atoms, provide explanations for phenomena associated with certain kinds of electromagnetic radiation. The energy of an individual photon is quantized, and the higher the photon's frequency, the greater its energy. This relationship is given by Planck's equation E = hν. Here E is the photon's energy and v is the photon's frequency. And h is the Planck constant. For example, a single gamma-ray photon carries close to about 100,000 times the energy of a visible light photon.
The effects of electromagnetic radiation on biological systems (and on many other chemical systems at standard temperature and pressure) depend on the radiation's power and frequency. For visible light or electromagnetic radiation with longer wavelengths, damage to cells and other materials is determined mainly by power, and results from heating by the combined energy of many photons. In contrast, with ultraviolet or higher-energy electromagnetic radiation, chemical substances and living cells suffer far greater damage than simple heating would cause. This is because, for high-energy photons, individual photons directly affect molecules.
Category
- Radio waves
- Infrared
- Visible light
- Ultraviolet
- X-rays
- Gamma rays