The Tools of the Trade. Electromagnetic Radiation

The astronomical census presented in chapter 1 has been made possible by the collection of electromagnetic radiation from space. Astronomy, by its very nature, is an observational science, not an experimental one. As much as they would like to, astronomers are not free to journey to these far distant objects in the exotic depths of the Universe and set up experiments to learn their secrets. Instead, they must rely on collecting the radiation which has been released by these celestial bodies.

The detection of electromagnetic radiation is, by far, the most advanced of our methods for examining the Universe. Since the dawn of human existence, mankind has done this quite naturally, since our eyes are really rather sophisticated detectors of visible electromagnetic radiation. Technologically, the study of visible light began in 1609 when Galileo used a telescope to look at the night sky. Today, a plethora of collecting and detecting devices is capable of sampling the radiation from many areas of the electromagnetic spectrum.

Electromagnetic Radiation. In the same way that the very fact of the Universe’s existence has caused many to wonder about it, so the nature of light has also puzzled mankind throughout history. Plato, Aristotle and Pythagoras all wondered about it but nothing really came of their pontifications.

Between the 1300s and 1600s, European researchers concentrated on the development of lenses and mirrors without really wondering what constituted the light they were studying. Whilst doing this, however, many became aware of the way in which rays of light moved through the air and interacted with other rays of light. Gradually, curiosity was raised and the physical nature of light was pondered.

In 1665, whilst attempting to understand a series of observations which generated the phenomenon of diffraction, Robert Hooke proposed that light is a rapid vibration of the medium through which it is passing. Hooke’s presentation marks the beginning of the wave theory of light which is still in use today.

Many excellent contributions were made to the burgeoning science of optics, but possibly the next two greatest milestones were the proof that light travelled with a finite velocity and that it was a type of electromagnetic wave. Both conclusions were finally and irrefutably reached in the middle of the nineteenth century.

From the days of Galileo, scientists had been trying to measure the speed of light. Those early attempts now seem ridiculously crude and their failure to show any time delay in light’s propagation from one location to another led some to believe that it travelled infinitely fast. In 1675, however, Danish astronomer Ole Christensen Römer observed an astronomical phenomenon which he could only explain if the speed of light were finite.

He was trying to measure the orbital period of Jupiter’s moon lo, and kept obtaining differing results. It was the first direct evidence that light did not propagate instantaneously and it even yielded a crude estimate for its velocity. Just over fifty years later, another phenomenon, known as aberration, was discovered by the Englishman James Bradley. This, too, was explainable only in terms of light travelling with a finite speed. Like Römer before him, Bradley also calculated a speed for light.

The first relatively accurate measurement of the speed of light was made by Frenchman Armand Hippolyte Louis Fizeau in the suburbs of Paris during 1849. His calculated figure was 315,000 km/s which, compared with today’s accepted figure of 299,792 km/s, is pretty good for the technology of the day.

At about the same time as these measurements in optics were taking place, another unrelated field of physical study was also making progress in leaps and bounds. Michael Faraday had applied his great mind to the study of electricity and magnetism. Within Faraday’s lifetime, Hans Christian Oersted had established a link between electricity and magnetism when, in 1820, he noticed that an electric current in a wire affects a magnetic compass needle. Faraday himself noticed a link between light and electromagnetism when he discovered that a strong magnetic field could alter the properties of a light ray.

Building upon this information and the collected results of many other experimentalists another physicist, James Clerk Maxwell, expertly extended the work and eventually distilled the behaviour of electromagnetism into a series of four theoretical equations.

His work stands today as one of the greatest pieces of theoretical physics ever, second only to the general theory of relativity. In the course of his investigations, Maxwell became aware that his equations predicted that electromagnetism could theoretically travel in the form of transverse waves. Solving his equations to give the speed of the wave, Maxwell discovered that it travelled at Fizeau’s measured speed of light.

This was so big a coincidence that Maxwell felt compelled to draw the conclusion that light was an electromagnetic disturbance propagated by a wave (see Fig. 2. 1). Not only that, but light was concentrated into such a restricted set of wavelengths that other types of electromagnetic radiation (with greater and smaller wavelengths) could be predicted to exist. In 1888, experimentalist Heinrich Rudolf Hertz dramatically proved Maxwell’s theory about electromagnetic radiation by producing the first radio waves.

Fig. 2. 1. An electromagnetic wave consists of two disturbances which propogate through space. The electric vector is an oscillating disturbance and is at right angles to a similarly oscillating magnetic disturbance. Together they travel through space at the speed of light

Today we split electromagnetic radiation into seven man-made divisions: radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. Fundamentally, there is no difference between these radiations; they simply have different wavelengths and frequencies (see Fig. 2. 2).

Fig. 2. 2. The electromagnetic spectrum. All types of electromagnetic wave are similar to one another; all that separates them is their wavelength, which dictates how the wave interacts with matter. 1 nanometre (nm) = 10Ä. Man-made divisions have been drawn to reflect these differences, although the transition from one type of electromagnetic wave to another is in reality smoothly continuous. (Adapted from Kaufmann, W.J., Universe, W.H. Freeman, 1987.)

The defining properties of the wave, namely the wavelength, X, and the frequency, f, are interlinked by the speed of the wave’s propagation, c = f λ. (2.1). In a vacuum, that speed is approximately 3 x 10⁸m/s.