Electromagnetic Radiation
Almost all our information in astronomy is obtained by the electromagnetic radiation traveling from the object to the observer. Apart from those few objects within the Solar System that we have been able to investigate directly, cosmic rays, neutrinos, in
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Electromagnetic Radiation
Introduction Almost all our information in astronomy is obtained by the electromagnetic radiation traveling from the object to the observer. Apart from those few objects within the Solar System that we have been able to investigate directly, cosmic rays, neutrinos, in due course, gravity waves and just possibly (if they exist) dark matter and dark energy are the only other information carriers likely to tell us about the universe as a whole. The electromagnetic radiation wave consists of a magnetic wave and an electric wave whose directions are orthogonal and which vary sinusoidally. The frequency of the sinusoidal variation is called the frequency of the wave and denoted usually by n. The separation of successive crests or troughs gives the wavelength, l. The product of n and l gives the wave’s velocity. In a vacuum this has a constant value, denoted by c, of 299 792 500 m s1. ln ¼ c
(7.1)
In other media the velocity is reduced from c by an amount given by the refractive index, m, of the material: v ¼
c m
(7.2)
Since even materials normally regarded as opaque have less than an infinite absorption, light penetrates them to a certain extent, and so they have a refractive index. We can thus talk, for example, about the velocity of light in a brick! In a medium where the electromagnetic radiation has a velocity less than c, the frequency remains constant, so from (7.1), with v replacing c, the wavelength becomes shorter.1
1
An effect used by oil immersion optical microscopes to increase their resolution.
C. R. Kitchin, Telescopes and Techniques, Undergraduate Lecture Notes in Physics, 145 DOI 10.1007/978-1-4614-4891-4_7, # Springer Science+Business Media New York 2013
146
7 Electromagnetic Radiation
As is well known, the special theory of relativity postulates that the velocity of electromagnetic radiation in a vacuum is the maximum possible for any normal particle, and experiments have confirmed this many times. However, because of the reduction in velocity given by (7.2), in any medium other than a vacuum it is possible for, say, a particle to exceed the local velocity of light. When the particle is charged, as for example is the case with most cosmic ray particles, a type of radiation is produced known as Cˇerenkov radiation. This is the electromagnetic equivalent of the sonic boom of a supersonic aircraft. It is of significance in that it results in noise spikes in CCD and other detectors (see Chap. 10).
Intensity The equation for the electric component of an electromagnetic wave is Eðx; tÞ ¼ E0 sin
2px þ 2ptn þ f l
(7.3)
where E(x,t) is the magnitude of the electric vector at position x and time t; Eo is the amplitude of the wave and f is the phase at t ¼ 0, x ¼ 0. The intensity of the radiation is given by Eo2. It has units of energy per unit area and per unit wavelength or frequency interval, and for convenience various different units are used over the spectrum. Thus, in the radio region, the units are janskys (1 Jy ¼ 1026 W m2 Hz1) while at shorter wavelength
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