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Facts of the Matter Richard Brill |
Illuminating the mystery
of light was no easy task
"We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."'
James Clerk Maxwell (1831-1879)Understanding light has been one of man's greatest and most recent accomplishments, its nature having been debated since the time of the ancient Greek philosophers.
Plato and his followers supposed that vision was produced by rays of light that originate in the eye and then strike the object being viewed, an erroneous view perpetuated today with Superman's X-ray vision emanating from his eyes in the comics.
Plato's student Aristotle disagreed, concluding that light travels in waves, like sound, because he saw the harmony of sound and blending of colors to be synonymous.
In the late 17th century Isaac Newton, whose experiments with prisms gave us our modern concept of color, postulated that light was a stream of particles that he called "corpuscles" because of the way it bends and separates into the rainbow of colors when passing through a glass prism.
Around the same time Christian Huygens, a renowned Dutch scientist, presented a serious challenge to Newton's "corpuscular" theory when he theorized that light results from vibrations in the corpuscles of luminous material, but the vibrations were transmitted through an "ether" like ripples in water.
The "wave vs. particle" debate continued for a century, until 1801, when Thomas Young, an English physician, performed an experiment that proved beyond a doubt that light is transmitted in the form of waves. The light and dark band visible in his famous "double slit" experiment could only be explained by the interference of light waves passing through the two slits.
Young also showed that Newton's experiments with the prism could be explained in terms of a simple wave theory of light. This conclusion was strongly attacked by some scientists in England in defense of Newton's demigod status, but it entrenched the idea of the wavelike nature of light that would continue for another century.
The invention of the battery in 1800, by Allesandro Volta, energized the discovery of important relationships between electricity, physics and chemistry that would shed much light on the mystery of light.
Many of these discoveries were made by Michael Faraday, the son of a blacksmith, who was apprenticed at the age of 14 to a bookbinder.
Many of the books that Faraday read while learning the trade just happened to be science books and he taught himself the basics of physics and chemistry.
At age 20, Faraday became the assistant to the great chemist Humphrey Davy. Spurred by the introduction of a revived atomic theory by John Dalton in 1803, Davy had been one of the first to use the newly discovered battery in his chemical studies, and he taught Faraday the secrets of the new chemistry.
Although Faraday's early work was in chemistry, he became interested in the relationship between electricity and magnetism after learning of the discovery in 1820 that a compass needle deflects when an electric current is switched on or off in a nearby wire.
In 1831, Faraday discovered magnetic induction when he sent a current through a coil of wire, creating a magnetic field that induced current in a second coil. This discovery led to his invention of the dynamo and ultimately the development of electric motors and generators, and forms the foundation of modern electromagnetic theory and technology.
Not being schooled in the mathematics of his era, Faraday expressed the induction relationship in terms of the number of "lines of flux emanating from charged bodies" that were cut, as he visualized the surrounding space as filled with a "field" of electric and magnetic forces.
In 1873 James Clerk Maxwell, a Scottish physicist and mathematician, formulated a unified description of electricity and magnetism by considering that Faraday's fields existed in Huygens' ether and behaved like an elastic medium. He derived a series of four differential equations, known simply as "Maxwell's equations," that completely describe the relationships that Faraday discovered and giving birth to formal field theory, the mainstay of today's physics.
One outcome of Maxwell's equations was his discovery that an oscillating electric charge should produce waves of electromagnetism that would travel through space emanating from the "waving" charge.
In 1888 Heinrich Hertz conducted a famous experiment in which he generated and detected these waves and went on to show that light has similar properties and was thus an electromagnetic phenomenon.
From Hertz's work came the understanding that light is part of the broad electromagnetic spectrum that includes radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma rays. It was soon discovered that the only difference in these various kinds of radiation was in their wavelength and frequency, with radio waves having the longest wavelength and lowest frequency.
Maxwell's science plays another important role in the understanding of light in the form of "statistical mechanics," a calculus that was worked out by a long line of famous physicists, including Maxwell, who were studying that branch of physics known as thermodynamics, dealing with the properties of heat and its transfer.
Statistical mechanics allows us to understand the behavior of incredibly large numbers of particles, such as the molecules in a volume of gas. It was first developed as a way of connecting Newton's laws and the gas laws, which describe how gases respond to changes in temperature, pressure and volume, and was eventually extended to liquids and solids as well.
Molecules are constantly in motion, the speed of their motion depending on the temperature. Like the chaotic motion of balls on the pool table after the break, not all of the molecules are moving at the same speed, but do show a statistical distribution of speeds not unlike the classical "bell curve." Although the speeds of individual balls change as they collide and is individually unpredictable, the distribution of speeds among the group of balls is regular and statistically predictable.
Maxwell's electromagnetic theory, when combined with statistical mechanics, predicted that the light emitted by a hot object could be due to the oscillations of electric charges of the molecules in the object.
Two important empirical laws in the form of equations that attempted to predict the spectrum of light based on temperature, were introduced, one by Wien in 1896, the other by Rayleigh in 1900, both of whom later would receive Nobel prizes for their work.
Both Wein's law and Rayleigh's law correctly explained the cause of the radiation, but neither correctly predicted the observed intensities of the various colors in the spectrum of emitted light, which is similar in shape to the "bell curve" of molecular speeds.
Wien's law was accurate at short wavelengths but deviated at longer wavelengths. The opposite was true of Rayleigh's.
That problem was solved when Max Planck, a German physicist who had studied statistical mechanics as part of his work in thermodynamics, introduced in 1900 what some historians consider to be the most radical idea in all of physics.
Planck announced his revolutionary idea that the EM energy emitted by a hot object could only take on certain values that he called "quanta." Planck's quantum of energy was proportional to the frequency of the light and a constant, now called Planck's constant. Planck received the Nobel prize for his work in 1918.
Planck's work marked a turning point in the history of physics, but the importance of his discovery was not appreciated until Einstein's explanation of the photoelectric effect in 1905, which marked the beginning of modern quantum mechanics, and for which he was awarded the Nobel prize in 1921.
The photoelectric effect refers to the ejection of electrons from the surface of a metal in response to incident light. The energy of the light is absorbed by electrons within the metal, giving the electrons sufficient energy to be "knocked" out of the metal.
Using Maxwell's wave theory of light, the energy of ejected electrons should increase with the intensity of the incident light, but this is not what happens. Instead, the energy of the emitted electrons does not depend at all on the intensity of the incident radiation but rather on its color or frequency.
In 1905, Einstein resolved the paradox by bringing Planck's quantum into the picture, proposing that light in general consisted of individual quanta, called photons, that interacted with the electrons in the metal like discrete particles, rather than as continuous waves. The energy of each photon was, as in Planck's explanation of the luminosity, proportional to the frequency and Planck's constant.
In Einstein's model, the intensity or brightness of the light was a measure of the number of incidents, while the energy of each photon, and the ejected electrons, remained the same as long as the frequency of the radiation was held constant.
Resolving the wave-particle duality, the issue of how light could travel as a wave and interact with matter as a particle, occupied the minds of the world's greatest scientists for two decades, finally coming to resolution in the late 1920s with the beginnings of what we now call "quantum theory."
Illuminating the mystery of the nature of light required the unification of many different kinds of studies in mechanics, optics, electricity, chemistry, heat and mathematics and has led ultimately to such diverse products as fluorescent lights, plasma TVs, LEDs, liquid crystals and chemical light sticks, to name a few.
The story of light is one of the best examples of the unity of the physical world and the strength of the theories that we have developed to understand it, however contrary to common sense it may seem.
But if you don't understand quantum theory, don't feel bad. It has been said if anyone claims to be able to think about quantum problems without getting giddy, it only shows they have not understood the first thing about it.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu