Light can be narrowly defined as the visible portion of the electromagnetic spectrum. A broader definition would include infrared, ultraviolet, and x-ray wavelengths, which are not visible to the eye. The nature of light has been the subject of controversy for thousands of years. Even today, while scientists know how light behaves, they do not always know why light behaves as it does.
The Greeks were the first to theorize about the nature of light. Led by the scientists Euclid and Hero (first century A.D.), they came to recognize that light traveled in a straight line. However, they believed that vision worked by intromission—that is, that light rays originated at the eye and traveled to the object being seen. Despite this erroneous hypothesis, the Greeks were able to successfully study the phenomena of reflection and refraction and derive the laws governing them. In reflection, they learned that the angles of incidence and reflection were approximately equal; in refraction, they saw that a beam of light would bend as it entered a denser medium (such as water or glass) and bend back the same amount as it exited.
The next contributor to the embryonic science of optics was the Arab mathematician and physicist Alhazen (965-1039), who is sometimes called the greatest scientist of the Middle Ages. Experimenting around the year 1000, he showed that light comes from a source (the Sun) and reflects from an object to the eyes, thus allowing the object to be seen. He also studied mirrors and lenses and further refined the laws of reflection and refraction.
By the twelfth century, scientists felt they had solved the riddles of light and color. The English philosopher Francis Bacon (1561-1626) contended that light was a disturbance in an invisible medium which could be detected by the eye; subsequently, color was caused by objects "staining" the light as it passed. More productive research into the behavior of light was sparked by the new class of realistic painters, who strove to better understand perspective and shading by studying light and its properties.
In the early 1600s, the refracting telescope was perfected by Galileo and Johannes Kepler, providing a reliable example of the laws of refraction. These laws were further refined by Willebrord Snel, whose name is most often associated with the equations for determining the refraction of light. By the mid-1600s, enough was known about the behavior of light to allow for the formulation of a wide range of theories.
The renowned English physicist and mathematician Isaac Newton was intrigued by the so-called "phenomenon of colors"—the ability of a prism to produce colors from white light. It had been generally accepted that white was a single color, and that a prism could somehow combine white light with others to form a multicolored mixture. Newton, however, doubted this assumption. He used a second prism to recombine the rainbow spectrum back into a beam of white light; this showed that white light must be a combination of colors, not the other way around.
Newton performed his experiments in 1666 and announced them shortly thereafter, subscribing to the corpuscular (or particulate) theory of light. According to this theory, light travels as a stream of particles that originate from a bright source and are absorbed by the eye. Aided by Newton's reputation, the corpuscular theory soon became accepted throughout Great Britain and in parts of Europe.
In the European scientific community, many scientists believed that light, like sound, traveled in waves. This group of scientists was most successfully represented by the Dutch physicist Christiaan Huygens, who challenged Newton's corpuscular theory. He argued that a wave theory could best explain the appearance of a spectrum as well as the phenomena of reflection and refraction.
Newton immediately attacked the wave theory. Using some complex calculations, he showed that particles, too, would obey the laws of reflection and refraction. He also pointed out that, if truly a wave form, light should be able to bend around corners, just as sound does; instead it cast a sharp shadow, further supporting the corpuscular theory.
In 1660, however, Francesco Grimaldi examined a beam of light passing through a narrow slit. As it exited and was projected upon a screen, faint fringes could be seen near the edge. This seemed to indicate that light did bend slightly around corners; the effect, called diffraction, was adopted by Huygens and other theorists as further proof of the wave nature of light.
One piece of the wave theory remained unexplained. At that time, all known waves moved through some kind of medium—for example, sound waves moved through air and kinetic waves moved through water. Huygens and his allies had not been able to show just what medium light waves moved through; instead, they contended that an invisible substance called ether filled the universe and allowed the passage of light. This unproven explanation did not earn further support for the wave theory, and the Newtonian view of light prevailed for more than a century.
The first real challenge to Newton's corpuscular theory came in 1801, when English physicist Thomas Young discovered interference in light. He passed a beam of light through two closely spaced pinholes and onto a screen. If light were truly particulate, Young argued, the holes would emit two distinct streams that would appear on the screen as two bright points. What was projected on the screen instead was a series of bright and dark lines—an interference pattern typical of how waves would behave under similar conditions.
If light is a wave, then every point on that wave is potentially a new wave source. As the light passes through the pinholes it exits as two new wave fronts, which spread out as they travel. Because the holes are placed close together, the two waves interact. In some places the two waves combine (constructive interference), whereas in others they cancel each other out (destructive interference), thus producing the pattern of bright and dark lines. Such interference had previously been observed in both water waves and sound waves and seemed to indicate that light, too, moved in waves.
The corpuscular view did not die easily. Many scientists had allied themselves with the Newtonian theory and were unwilling to risk their reputations to support an antiquated wave theory. Also, English scientists were not pleased to see one of their countrymen challenge the theories of Newton; Young, therefore, earned little favor in his homeland.
Throughout Europe, however, support for the wave nature of light continued to grow. In France, Etienne-Louis Malus (1775-1826) and Augustin Jean Fresnel (1788-1827) experimented with polarized light, an effect that could only occur if light acted as a transverse wave (a wave which oscillated at right angles to its path of travel). In Germany, Joseph von Fraunhofer (1787-1826) was constructing instruments to better examine the phenomenon of diffraction and succeeded in identifying within the Sun's spectrum 574 dark lines corresponding to different wavelengths.
In 1850 two French scientists, Jéan Foucault and Armand Fizeau, independently conducted an experiment that would strike a serious blow to the corpuscular theory of light. An instructor of theirs, Dominique-Françios Arago, had suggested that they attempt to measure the speed of light as it traveled through both air and water. If light were particulate it should move faster in water; if, on the other hand, it were a wave it should move faster in air. The two scientists performed their experiments, and each came to the same conclusion: light traveled more quickly through air and was slowed by water.
Even as more and more scientists subscribed to the wave theory, one question remained unanswered: through what medium did light travel? The existence of ether had never been proven—in fact, the very idea of it seemed ridiculous to most scientists. In 1872, James Clerk Maxwell suggested that waves composed of electric and magnetic fields could propagate in a vacuum, independent of any medium. This hypothesis was later proven by Heinrich Rudolph Hertz, who showed that such waves would also obey all the laws of reflection, refraction, and diffraction. It became generally accepted that light acted as an electromagnetic wave.
Hertz, however, had also discovered the photoelectric effect, by which certain metals would produce an electrical potential when exposed to light. As scientists studied the photoelectric effect, it became clear that a wave theory could not account for this behavior; in fact, the effect seemed to indicate the presence of particles. For the first time in more than a century there was new support for Newton's corpuscular theory of light.
The photoelectric effect was explained by Albert Einstein in 1905 using the principles of quantum physics developed by Max Planck. Einstein claimed that light was quantized—that is, it appeared in "bundles" of energy. While these bundles traveled in waves, certain reactions (like the photoelectric effect) revealed their particulate nature. This theory was further supported in 1923 by Arthur Holly Compton, who showed that the bundles of light—which he called photons—would sometimes strike electrons during scattering, causing their wavelengths to change.
By employing the quantum theories of Planck and Einstein, Compton was able to describe light as both a particle and a wave, depending upon the way it was tested. While this may seem paradoxical, it remains an acceptable model for explaining the phenomena associated with light and is the dominant theory of our time.
See also Photon.
Born, Max, and Emil Wolf. Principles of Optics. New York: Pergamon Press, 1980.
Hecht, Eugene. Optics. Reading, MA: Addison-Wesley Publishing Company, 1987.