Electromagnetic radiation
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Electromagnetic radiation is a combination of oscillating electric and magnetic fields in perpendicular orientation to each other, moving through space, effectively transporting energy from one place to another. The visible region of the electromagnetic spectrum, ranges from about 400nm to 700nm. We perceive this region as light. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism.
Electromagnetic waves were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz.
Any electric charge which accelerates, radiates electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as waves or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10-34 J·s is Planck's constant, and ν is the frequency of the wave.
Generally, electromagnetic radiation is classified by wavelength into radio, microwave, infrared, the visible region, (we perceive as light,) ultraviolet, X-rays and gamma rays. The details of this classification are contained in the article on the electromagnetic spectrum.
The effect of radiation depends on the amount of energy per quantum it carries. Higher frequencies have shorter wavelengths, and longer wavelengths have lower frequencies. One rule is always obeyed, regardless of the circumstances. Radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity).
If the visible region of the electromagnetic spectrum shines on a bowl of fruit, (say the sun,) this results in our Visual perception identifying information from the scene. Our visual system processes the multitude of reflected frequencies into different shades and hues in our brain, and through the miracle called: the "psychophysical phenomenon," most humans perceive it as a bowl of fruit.
Alternatively, light can carry information. In particular through optical fiber. Although not suitable for direct viewing, it can carry data. That data can be translated as sound or even an image. It operates in a coded form similar to how radio waves functioned. Radio waves are used for carring information by varying amplitude and varying frequency within a frequency band.
Spectroscopy can detect a much wider region of the electromagnetic spectrum than the visible range of 400nm to 700nm. A common laboratory spectroscope can detect frequencies from 2nm to 2500nm. More in depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example; many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.
When electromagnetic radiation impinges upon a conductor it induces an electric current flow on the surface of that conductor. This effect (the skin effect) is used in antennas. Electromagnetic radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.
See also
External links
ca:Radiació electromagnètica
da:Elektromagnetisk stråling
de:Elektromagnetische Welle
es:Radiación electromagnética
fr:Rayonnement électromagnétique
nl:Elektromagnetische straling
ja:電磁波
pt:Radiação electromagnética
fi:Sähkömagneettinen säteily
sv:Elektromagnetisk strålning
