Other electromagnetic waves

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There are other electromagnetic waves which travel at the speed of light and turn out to have the same wave structure as light but which are produced in different ways and can have enormously different wavelengths, as indicated. Radio waves have the lowest energies and gamma rays the highest.

gamma rays x-rays uv

infrared microwaves visible infrared microwaves radio waves

visible wavelength

Spectum of electromagnetic waves.

We are all familiar with non-visible electromagnetic waves in certain contexts. Radio waves might be associated with radio and television, infrared waves with night vision binoculars, ultraviolet light with sunburn, microwaves with cooking, and X-rays and gamma rays with medical procedures.

8.5.1 Message from the stars

Jansky's antenna. Courtesy of National Radio Astronomy Laboratory/ AUI/NSF.

As we shall see in Chapter 10, electromagnetic waves are produced whenever an electric charge is accelerated. The universe is full of such radiation, ranging in wavelength right across the electromagnetic spectrum. It comes from natural sources and crosses billions of light years of interstellar space, carrying information about a variety of astrophysical phenomena.

Radio waves were the first cosmic radiations to be detected other than visible light.

Karl Guthe Jansky (1905-1950) was employed by Bell Telephone Laboratories as a radio engineer. He was asked to investigate a mysterious sort of static in the form of a steady hiss of unknown origin that was interfering with the short wave radio signals used by Bell Laboratories for their transatlantic radio telephone system. Jansky found that the hiss was coming from space — from the centre of our galaxy.

Jansky's discovery made front page reading in the New York Times of 5 May 1933 and NBC radio subsequently broadcast a recording of 'Jansky's star noise'. Despite the favourable publicity, Jansky was not permitted by his employers to continue his investigations into the extraterrestrial radio waves. Much later, in 1998, Bell Laboratories recognised his work by erecting the Karl Jansky monument on the site where the cosmic radio waves were first detected.

The accidental discovery of cosmic radio emissions was not really followed up until after the end of World War II,

Jansky's antenna. Courtesy of National Radio Astronomy Laboratory/ AUI/NSF.

Green Bank Telescope. Courtesy of National Radio Astronomy Laboratory/ AUI/NSF.

when a number of strong sources of cosmic radio waves were identified. The largest individual radio telescope operating today is a fully steerable 100-m-diameter dish operated by the National Radio Astronomy Observatory at Green Bank, West Virginia. More than 30 years later, and less than a mile from the site of Jansky's aerial, Penzias and Wilson first detected the cosmic microwaves which are the tell-tale remnants of the big bang (see Chapter 5).

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