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Electromagnetic radiation consists of discrete packets of energy, which we call

 photons. A photon consists of an oscillating electric field component, E, and an

oscillating magnetic field component, M. These interacting electric and magnetic

fields are at right angles to one another and also to the direction of propagation of 

the energy. Thus, an electromagnetic wave is a transverse wave.

If the direction of the electric field is constant, the wave is said to be polarized

(see polarization of light). Electromagnetic radiation does not require a materialmedium and can travel through a vacuum. The theory of electromagnetic radiation

was developed by James Clerk Maxwell and published in 1865. He showed that

the speed of propagation of electromagnetic radiation should be identical with that

of light, about 3 * 108 m/s. Subsequent experiments by Heinrich Hertz verified

Maxwell's prediction through the discovery of radio waves, also known as

Hertzian waves.

All photons (in a given, non-absorbing medium) travel at the same velocity, v.

The physical distance in the direction of propagation over which the electric and

magnetic fields of a photon make one complete oscillation is called thewavelength, of the electromagnetic radiation. The relationship between the lightvelocity, wavelength, and frequency is:

v =

The electromagnetic nature of all photons is the same, but photons can have

different frequencies. The energy, E, of one photon depends on its frequency of 

oscillation:

E = h = hv /Where, h is Planck's constant (6.62618x10-34 J·s).

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Radio frequency

Radio waves generally are utilized by antennas of appropriate size (according

to the principle of resonance), with wavelengths ranging from hundreds of meters

to about one millimetre. They are used for transmission of data, via modulation.

Television, mobile phones, wireless networking and amateur radio all use radiowaves. Radio waves can be made to carry information by varying a combination

of the amplitude, frequency and phase of the wave within a frequency band. EM

radiation may also cause certain molecules to absorb energy and thus to heat up,

thus causing thermal effects and sometimes burns; this is exploited in microwave ovens.

Microwaves

The super high frequency (SHF) and extremely high frequency (EHF) of 

microwaves come next up the frequency scale. Microwaves are waves which aretypically short enough to employ tubular metal waveguides of reasonable

diameter. Microwaves are absorbed by molecules that have a dipole moment in

liquids. In a microwave oven, this effect is used to heat food. Low-intensity

microwave radiation is used in Wi-Fi, although this is at intensity levels unable to

cause thermal heating.

Infrared radiation

The infrared part of the electromagnetic spectrum covers the range from

roughly 300 GHz (1 mm) to 400 THz (750 nm). This radiation is typically

absorbed by so-called rotational modes in gas-phase molecules, by molecular 

motions in liquids, and by phonons in solids.

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Hot objects ( black-body radiators) can radiate strongly in this range. It is

absorbed by molecular vibrations, where the different atoms in a molecule vibrate

around their equilibrium positions. This range is sometimes called the fingerprint

region since the mid-infrared absorption spectrum of a compound is very specific

for that compound.

Visible spectrumAbove infrared in frequency comes visible light. This is the range in which the

sun and stars similar to it emit most of their radiation. It is probably not a

coincidence that the human eye is sensitive to the wavelengths that the sun emits

most strongly. Visible light (and near-infrared light) is typically absorbed and

emitted by electrons in molecules and atoms that move from one energy level to

another. A rainbow shows the optical (visible) part of the electromagneticspectrum.

Ultraviolet Next in frequency comes ultraviolet (UV). This is radiation whose wavelength

is shorter than the violet end of the visible spectrum, and longer than that of an x-

ray. Being very energetic, UV can break chemical bonds, making moleculesunusually reactive or ionizing them, in general changing their mutual behaviour.

Sunburn, for example, is caused by the disruptive effects of UV radiation on skin 

cells, which can even cause skin cancer , if the radiation irreparably damages the

complex DNA molecules. The Sun emits a large amount of UV radiation, which

could quickly turn Earth into a barren desert; however, most of it is absorbed by

the atmosphere's ozone layer before reaching the surface.

X-rays

After UV come X-rays. Hard X-rays have shorter wavelengths than soft X-

rays. As they can pass through most substances, X-rays can be used to 'see

through' objects, most notably bodies (in medicine), as well as for high-energy

 physics and astronomy. Neutron stars and accretion disks around black holes emit

X-rays, which enable us to study them. X-rays are given off by stars, and strongly

 by some types of nebulae.

Gamma rays

After hard X-rays come gamma rays, which were discovered by Paul Villard in1900. These are the most energetic photons having no defined lower limit to their 

wavelength. They are useful to astronomers in the study of high energy objects or 

regions and find a use with physicists thanks to their penetrative ability and their 

 production from radioisotopes. The wavelength of gamma rays can be measuredwith high accuracy by means of Compton scattering.