# Definition

A disturbance that can travel through a vacuum as well as through a material medium, light and radio waves being familiar forms. It consists of oscillating (time-varying) electric and magnetic fields with directions at right angles to each other and to the direction of propagation. The two fields are bound together, the time-varying electric and magnetic components regenerating each other in an endless cycle that moves from one point to the next through space. The radiation transfers energy and also momentum. It travels through a vacuum at the speed of light, c, which is of 299,792.58kms-1 (186,201 miles/s). The speed is slightly reduced on entering a medium, such as air, glass or water.

Electromagnetic radiation is caused by the acceleration of charged particles, such as electrons. Its propagation through space can be fully described in terms of wave motion. Like other periodic waves, electromagnetic waves have a wavelength l and a frequency v, which are related by lv = c. Reflection, refraction, interference, and polarization can be explained in terms of wave motion. When radiation interacts with matter, however, it exhibits particle-like behaviour, as when it undergoes absorption or emission. It thus has a dual wave-particular nature. A particle nature was originally proposed by Newton but in its present form the concept is part of quantum theory. Light and other kinds of radiation interact with matter as quanta. A quantum of radiation of frequency v transfers energy hv and moment hv/c2, where h is Planck constant. The quantum of electromagnetic radiation is the photon. Light, etc., is thus absorbed by or emitted from atoms or molecules in the form of photons.

The range of frequences ( or wavelengths) of electromagnetic radiation is known as the electromagnetic spectrum. The spectrum can be divided into various regions range from low-frequency low-energy radio waves through infrared radiation, light (Violet l = 3.9x10-7m and Red l = 7.8x10-7m), ultraviolet radiation, X-rays, to high-frequency high-energy gamma rays.

# General Properties

 Can travel through empty space Speed of light is constant in space. All forms of light have same speed in space. Wavelength of light defined similarly to that of water waves - distance between crests or troughs.

# Inverse Square Law for Magnetic Waves

Consider an area of 1m2 which forms part of the surface of a sphere of radius 1m receives wave energy from a point source placed at the centre S.

If P is the energy passing through this unit area in joules per second, i.e., if P is power in watts, then we say that P is the wave intensity in Wm-2.

If the distance is increased to 2m it is clear from the geometry of the figure that the same power is now spread over an area of 22 = 4m2. Consequently the wave intensity is reduced to . Similarly, when the distance is increased to 3m the wave intensity in Wm-2 is now only .

Wave intensity is defined as the power transmitted per unit area of the wavefront.

In general, the wave intensity at any distance x from a point source is given by .

This relationship between wave intensity and distance is expressed by the inverse square law.

The electromagnetic wave intensity from one point source in free space is inversely proportional to the square of the distance from the source.

Light as a special case. The measurement of light wave energy (photometry) is complicated by the fact that light sources contain non-visible radiation as well, example ultraviolet (UV) and infrared (IR). Special units called lux (not Wm-2) therefore have to be used, but all the same, the inverse law still holds.

# Electromagnetic Spectrum

The distribution of this phenomena is obtained when electromagnetic radiation is absorbed or emitted. Well-known examples of this phenomena are when a white light passes through a prism or the rainbow. This effect is also obtained when white light passes through a diffraction grating, which produces a continuous spectrum, in which all wave-lengths (between certain limits) are present.

Type of Radiation Frequency Range (Hz) Wave-length (l) Range Type of Transition

< 3x1011 > 1mm nuclear spin flips*
microwaves 3x1011 - 1013 1mm-25µm molecular rotations, electron spin flips *
 infrared visible ultaviolet
 1013 - 1014 4 - 7.5x1014 1015 - 1017
 25µm-750nm 750nm-400nm 400nm-1nm
 molecular vibrations outer electron outer electron
x-rays 1017 - 1020 1nm-1pm inner electron
gamma-rays 1020 - 1024 <10-12m nuclear

*energy levels split by a magnetic field.

In 1931 in the United States, Karl Jansky (1905-50), a radio engineer, had discovered the radio waves from space. This discovery happened when he was investigating the nature of static, when he found that he was picking up emissions from the sky. He tracked them down to the Milky Way. He published few papers, but never followed the subject through. Another American, G. Reber, before the World War II, had setup a dish-shaped radio telescope and made the first radio map of the Milky Way. A British team led by J.S. Hey, during the war, found that the radars where not jammed by transmittions from Germany, as it was previously thought, but by radio waves from the Sun.

Subsequently, radio telescopes were set up and a new branch of science was well under way. The Earth is close to the Sun and for this it is obtrusive, since the Sun is a radio source but cosmic standards not a powerful one. Even Jupiter is known as to be a source of radio waves. But most radio sources lie far beyond the Solar System. Those in our Galaxy include many supernova remnants, of which the Crab Nebula is the most celebrated example. There are many radio galaxies at greater distances, which are extremely power at long length still not known.

Radio astronomy had help us to tremendously to increase our knowledge of the universe. Nothing or almost nothing would have been known about pulsars, which are neutron stars, or quasars, which are extragalatic and may well be the most powerful objects known to man, without it. More over, radio waves have been studied for more time from the visible light waves, so that our information about most distinct regions of the universe is derived entirely from radio work.

The infrared lights are absorbed by the upper atmosphere and are studied by a special equipment carried by satellites. However, some of it passes through some hole in the atmosphere and therefore, they can be studied from ground. This branch of research was useful to increase our knowledge about stellar evolution. Infrared is also produced by extremely young stars, such as the variable V1057 in Cygnus, which when the dust, contained in the clouds surrounding the star, is heated. The Brecklin's Object inside the Orion Nebula is an example of some objects that are detectable only by infrared techniques.

## Ultraviolet, g- and x-rays

Even these rays are absorbed by the upper atmosphere. To study high-energy Ultraviolets, x-rays and gamma-rays (that is, radiations below 2,900A) a special equipment, carried by a satellite or a rocket, must be used.

Copernicus and the International Ultraviolet Explorer, satellites, have studied radiation from the rare very hot stars - at temperatures of 30,000°C (54,000°F or 30,273K) or more. The gas clouds encountered, and passed through, by the ultraviolet light effect it and proportions of differnt chemical elements in the gas in space where revealed in detail.

X-ray astronomy satellites, like Uhuru (launched in 1970), HEAD-1 and the Einstein Observatory(1978-81), have detected many powerful x-ray sources. One member, of the double stars in our galaxy, is a very compact neutron star, which is pulling gases off its ordinary companion: the hot gas steam emits x-rays copiously.

The first pictures of x-ray sources where taken by the sophisticated Einstein Observatory. These pictures revealed the distribution of extremely hot x-ray emitting gas in the remains of exploded stars, and between the galaxies in very distant clusters of galaxies. Giant gas clouds in our galaxy emit gamma- radiation, this was found by the Gamma-ray Observatories. Short bursts of gamma-rays from space, where discovered by gamma-ray satellites launched for military surveillance of the Earth.