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Measuring Light

Dutch astronomer, law of refraction, light quantum, particle theory, thermal radiation

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The early modern scientists Galileo, Johannes Kepler of Germany, and Rene Descartes of France all made contributions to the understanding of light. Descartes discussed optics and reported the law of refraction in his famous Discours de la methode (Discourse on Method), published in 1637. The Dutch astronomer and mathematician Willebrord Snell independently discovered the law of refraction in 1620, and the law is now named after him.

During the late 1600s, an important question emerged: Is light a swarm of particles or is it a wave in some pervasive medium through which ordinary matter freely moves? English physicist Sir Isaac Newton was a proponent of the particle theory, and Huygens developed the wave theory at about the same time. At the time it seemed that wave theories could not explain optical polarization because waves that scientists were familiar with moved parallel, not perpendicular, to the direction of wave travel. On the other hand, Newton had difficulty explaining the phenomenon of interference of light. His explanation forced a wavelike property on a particle description. Newton’s great prestige coupled with the difficulty of explaining polarization caused the scientific community to favor the particle theory, even after English physicist Thomas Young analyzed a new class of interference phenomena using the wave theory in 1803.

The wave theory was finally accepted after French physicist Augustin Fresnel supported Young’s ideas with mathematical calculations in 1815 and predicted surprising new effects. Irish mathematician Sir William Hamilton clarified the relationship between wave and particle viewpoints by developing a theory that unified optics and mechanics. Hamilton’s theory was important in the later development of quantum mechanics.

Between the time of Newton and Fresnel, scientists developed mathematical techniques to describe wave phenomena in fluids and solids. Fresnel and his successors were able to use these advances to create a theory of transverse waves that would account for the phenomenon of optical polarization. As a result, an entire wave theory of light existed in mathematical form before British physicist James Clerk Maxwell began his work on electromagnetism. In his theory of electromagnetism, Maxwell showed that electric and magnetic fields affect each other in such a way as to permit waves to travel through space. The equations he derived to describe these electromagnetic waves matched the equations scientists already knew to describe light. Maxwell’s equations, however, were more general in that they described electromagnetic phenomena other than light and they predicted waves throughout the electromagnetic spectrum. In addition, his theory gave the correct speed of light in terms of the properties of electricity and magnetism. When German physicist Gustav Hertz later detected electromagnetic waves at lower frequencies, which the theory predicted, the basic correctness of Maxwell’s theory was confirmed.

Maxwell’s work left unsolved a problem common to all wave theories of light. A wave is a continuous phenomenon, which means that when it travels, its electromagnetic field must move at each of the infinite number of points in every small part of space. When we add heat to any system to raise its temperature, the energy is shared equally among all the parts of the system that can move. When this idea is applied to light, with an infinite number of moving parts, it appears to require an infinite amount of heat to give all the parts equal energy. But thermal radiation, the process in which heated objects emit electromagnetic waves, occurs in nature with a finite amount of heat. Something that could account for this process was missing from Maxwell’s theory. In 1900 Max Planck provided the missing concept. He proposed the existence of a light quantum, a finite packet of energy that became known as the photon.

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