|
[RadCCORE Wiki Home Page :: Definitions :: This page]
|
electromagnetic radiation |
'''Electromagnetic radiation''' is generally described as a self-propagating
wave
in space with
electric and
magnetic components. These components oscillate at right angles to each other and to the direction of propagation, and are in
phase with each other. Electromagnetic radiation is classified into types according to the
frequency of the wave: these types include, in order of increasing frequency,
radio waves,
microwaves,
infrared radiation,
visible light,
ultraviolet radiation,
x-rays and
gamma rays.
EM radiation carries energy and momentum, which may be imparted when it interacts with matter.
EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). However, these characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radiation is measured over relatively large timescales and over large distances, and the particle characteristics are evident when measuring small distances and timescales. Both characteristics have been confirmed in a large number of experiments.
===Wave model===
An important aspect of the nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually has a spectrum of frequencies which sum together to form the resultant wave. Different frequencies undergo different angles of refraction.
A wave consists of successive troughs and crests, and the distance between two adjacent crests is called the wavelength. Waves of the electromagnetic spectrum vary in size, from very long radio waves the size of buildings to very short gamma rays smaller than atom nuclei. Frequency is the inverse of wavelength, according to the equation:
:
where ''v'' is the speed of the wave (''c'' in a vacuum, or less in other media), ''f'' is the frequency and λ is the wavelength. As waves cross boundaries between different media, their speed changes but their frequency remains constant.
Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference.
The energy in electromagnetic waves is sometimes called radiant energy.
=== Particle model===
In the particle model of EM radiation, a wave consists of discrete packets of energy, or quanta, called photons. The frequency of the wave is proportional to the magnitude of the particle's energy. Moreover, because photons are emitted and absorbed by charged particles, they act as transporters of energy.
As a photon is absorbed by an atom, it excites an electron, elevating it to a higher energy level. If the energy is great enough, so that the electron jumps to a high enough energy level, it may escape the positive pull of the nucleus and be liberated from the atom in a process called ionization. Conversely, an electron that descends to a lower energy level in an atom emits a photon of light equal to the energy difference.
Since the energy levels of electrons in atoms are discrete, each element emits and absorbs its own characteristic frequencies.
Together, these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium absorbing different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, dark bands in the light emitted by a distant star are due to the atoms in the star's atmosphere. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons. This is manifested in the emission spectrum of nebulae. Today, scientists use this phenomenon to observe what elements a certain star is composed of. It is also used in the determination of the distance of a star, using the so-called red shift.
===Speed of propagation===
Any electric charge which accelerates, or any changing magnetic field, produces 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 a wave 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.
One rule is always obeyed regardless of the circumstances: EM 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.)
In a medium (other than vacuum), velocity of propagation or refractive index are considered, depending on frequency and application. Both of these are ratios of the speed in a medium to speed in a vacuum.
== Electromagnetic spectrum ==
main|electromagnetic spectrum
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.
|
|
Last modified: 13.09.07 by jross
|
DHTS Web Services