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10.3 The Nature of Light

What is the nature of light? Experiments in the 18th century showed that it is related to the forces of electricity and magnetism. This sounds unlikely, so it is worth recounting how physicists came to this realization. The understanding of light depended on a series of elegant experiments carried out by the English physicist Michael Faraday, and an important theory proposed by the Scottish physicist James Clerk Maxwell.

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James Clerk Maxwell. Click here for original source URL


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Michael Faraday. Click here for original source URL

Michael Faraday was a bookbinder's apprentice who rose to become the director of the most prestigious scientific organization in science, the Royal Institution in London. He was a masterful experimenter and a wonderful communicator. Every Christmas he would deliver a public lecture series crammed with demonstrations, a series that continues in his name today. Faraday discovered that a changing electric field could create a magnetic force. Once, the Prime Minister visited his lab and complained about abstract research and the lack of any use for all the gizmos in the lab. Faraday replied that the results were important, so important that one day Her Majesty's government would tax their many applications. He was right. The connection between electric and magnetic forces is the basis for electric motors and the generation of all electric power.

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Drawing of Michael Faraday's 1831 experiment showing electro magnetic induction between coils of wire, using 19th century apparatus, from an 1892 textbook on electricity. On the right is a liquid battery that provides a current that flows through the small coil of wire(A) creating a magnetic field. When the small coil is stationary, no current is induced. However, when the small coil is moved in or out of the large coil (B), the change in magnetic flux induces a current in the large coil. This is detected by the deflection of the needle in thegalvanometer instrument (G) on the left. Click here for original source URL.


Faraday also knew that a changing magnetic field could generate an electric current. These two results suggest an intimate connection between electric and magnetic forces. Both are familiar to us in the everyday world; the current in an electric motor creates a magnetic force that drives a rotor, and water flowing though a turbine creates a changing magnetic force that generates electricity. But what do these forces have to do with light?

In the 19th century, Scottish physicist James Clerk Maxwell came up with his theory of electricity and magnetism. He derived an elegant set of equations linking the two forces. In the theory, a changing electric or magnetic force could generate a disturbance that behaved like a wave. Since the electric and magnetic changes are always linked, this disturbance is called an electromagnetic wave. Maxwell used the theory to predict that electromagnetic waves should travel through space at 300,000 kilometers per second. This is exactly the speed of light! Maxwell therefore speculated that light was just one form of electromagnetic radiation.

The word radiation must be used carefully. Radiation describes light, radio waves, X-rays, and the other types of electromagnetic waves. You should be aware that physicists and engineers speak of streams of subatomic particles, such as protons or electrons, as radiation. For example, the dangerous particles escaping from radioactive material are sometimes called radiation. Electromagnetic radiation has a general property that is of great interest: it carries energy and information from one place to another across the vast reaches of space.