Light is generally defined as that portion of the electromagnetic spectrum with wavelengths between 400 and 700 nanometers (billionths of a meter). Like all forms of electromagnetic radiation, light travels with a speed of 186,282 miles (299,728 kilometers) per second in a vacuum. It is perhaps the swiftest and most delicate form of energy found in nature.
Considering how important light is in our daily lives, it is hardly surprising that philosophers and scientists have been trying to understand its fundamental nature for centuries. The ancient Greeks, for example, worked out some of the basic laws involving light, including the laws of reflection (bouncing off an object) and refraction (bending through an object). They did so in spite of the fact that they started with only an incorrect concept of light. They believed that light beams started out in the human eye and traveled to an object.
Corpuscle: A particle.
Diffraction: The bending of light or another form of electromagnetic radiation as it passes through a tiny hole or around a sharp edge.
Duality: The tendency of something to behave in two very different ways, for example, as both energy and matter.
Electromagnetic spectrum: The whole range of radiation that travels through a vacuum with a speed of about 3 × 108 meters per second.
Ether: Also spelled aether; medium that was hypothesized by physicists to explain the wave behavior of light.
Photoelectric effect: The production of an electric current when a beam of light is shined on a metal.
Photon: A tiny package of light energy.
Wave: A regular pattern of motion that involves some kind of disturbance in a medium.
Wavelength: The distance between two successive identical parts of a wave, such as two crests or two troughs.
With the rise of modern physics in the seventeenth century, scientists argued over two fundamental explanations of the nature of light: wave versus particle. According to the particle theory of light, light consists of a stream of particles that come from a source (such as the Sun or a lamp), travel to an object, and are then reflected to an observer. This view of light was first proposed in some detail by Isaac Newton (1642–1727). Newton's theory is sometimes known as the corpuscular theory of light.
At about the same time, the wave theory of light was being developed. According to the wave theory, light travels through space in the form of a wave, similar in some ways to water waves. The primary spokesperson for this concept was Dutch physicist Christiaan Huygens (1629–1695).
Over time, the wave theory became more popular among physicists. One of the main reasons for the triumph of the wave theory was that many typical wave properties were detected for light. For example, when light passes through a tiny pinhole or around a sharp edge, it exhibits a property known as diffraction. Diffraction is well known as a property of waves among physicists. Almost anyone can witness the diffraction of water waves as they enter a bay or harbor, for example. If light exhibits diffraction, scientists thought, then it must be transmitted by waves.
Today, scientists usually talk about light as if it were transmitted by waves. They talk about the wavelength and frequency of light, both properties of waves, not particles.
One of the serious problems arising out of the wave theory of light is the problem of medium. Wave motion is the regular up-and-down motion of some material. For water waves, that material (or medium) is water. If light is a form of wave motion, scientists asked, what is the medium through which it travels?
The obvious answer, of course, is that light travels through air as a medium. But that answer is contradicted by the fact that light also travels through a vacuum, a region of space that contains no air or anything else.
To resolve this problem, scientists developed the concept of an ether (or aether). The ether was defined as a very thin material—perhaps like air, but much less dense—that permeates all of space. Light could be explained, then, as a wave motion in the ether.
Unfortunately, efforts to locate the ether were unsuccessful. In one of the most famous negative experiments of all time, two American physicists, Albert A. Michelson (1852–1931) and Edward W. Morley (1838–1923), devised a very precise method for detecting the ether. No matter how carefully they searched, they found no ether. Their experiments were so carefully designed and carried out that physicists were convinced that the ether did not exist.
Today, a somewhat simpler view of light as a wave phenomenon exists. Light is a form of radiation that needs no medium through which to travel. It consists of electric and magnetic fields that pulsate up and down as they travel through space.
By the early 1900s, most physicists had accepted the idea that light is a form of wave motion. But they did so somewhat reluctantly because some facts about light could not really be explained by the wave theory. The most important of these was the photoelectric effect.
The photoelectric effect was first observed by German physicist Heinrich Hertz (1857–1894) in about 1888. He noticed that when light is shined on a piece of metal, an electric current (a flow of electrons) is produced. Later experiments showed a rather peculiar property of the photoelectric effect. It doesn't make any difference how intense the light is that is shined on the metal. A bright light and a dim light both produce the same current. What does make a difference is the color of the light. Red light, for example, produces more of a current than blue light.
Unfortunately, there is no way for the wave theory of light to explain this effect. In fact, it was not until 1905 that a satisfactory explanation of the photoelectric effect was announced. That explanation came from German-born American physicist Albert Einstein (1879–1955). Einstein showed that the photoelectric effect could be explained provided that light were thought of not as a wave but as a bundle of tiny particles.
But the concept of light-as-particles is just what Isaac Newton had proposed more than 200 years earlier—and what physicists had largely rejected. The important point about Einstein's explanation, however, was that it worked. It explained a property of light that wave theory could not explain.
The conflict between light-as-waves and light-as-particles has had an interesting resolution. Today, physicists say that light sometimes acts like a wave and sometimes acts like a collection of particles. Perhaps it is a wave consisting of tiny particles. Those particles are now called photons. They are different from other kinds of particles we know of since they have no mass. They are just tiny packages of energy that act like particles of matter.
Two sets of laws are used to describe light. One set is based on the idea that light is a wave. Those laws are used when they work. The second set is based on the idea that light consists of particles. Those laws are also used when they work.
The philosophy of using wave or particle explanations for light is an example of duality. The term duality means that some natural phenomenon can be understood in two very different ways. Interestingly enough, other forms of duality have been discovered. For example, scientists have traditionally thought of electrons as a form of matter. They have mass and charge, which are characteristics of matter. But it happens that some properties of electrons can best be explained if they are thought of as waves. So, like light, electrons also have a dual character.
The Čerenkov effect (pronounced che-REN-kof) is the emission of light from something transparent when a charged particle travels through the material with a speed faster than the speed of light in that material. The effect is named for Russian physicist Pavel A. Čerenkov (1904–1990), who first observed it in 1934.
Many people have seen the Čerenkov effect without realizing it. In photographs of a nuclear power plant, the water surrounding the reactor core often seems to glow with an eery blue light. That light is Čerenkov radiation produced when rapidly moving particles produced in the core travel through the cooling water around it.
The definition of the Čerenkov effect often puzzles students because it includes references to charged particles traveling faster than the speed of light. Of course, nothing can travel faster than the speed of light in a vacuum. In a fluid such as air, water, plastic, or glass, however, it is possible for objects to travel faster than the speed of light. When they do so, they produce the bluish glow seen in a nuclear reactor.
In January 2001, scientists at two separate laboratories in Cambridge, Massachusetts, conducted landmark experiments in which they brought light particles to a halt and then sped them back up to their normal speed. In the experiments, the scientists created chambers that held a gas. One research team used sodium gas, the other used the gas form of rubidium, an alkaline metal element. The gases in both chambers were chilled magnetically to within a few millionths of a degree of absolute zero, or −459°F (−273°C). The scientists passed a light beam into the specially prepared chambers, and the light became fainter and fainter as it slowed and then eventually stopped. Even thought the light vanished, the information on its particles was still imprinted on the atoms of sodium and rubidium. That information was basically frozen or stored. The scientists then flashed a second light through the gas, which essentially reconstituted or revived the original beam. The light left each of the chambers with almost the same shape, intensity, and other properties it had when it entered the chambers.
Scientists believe the biggest impact of these experiments could come in futuristic technologies such as ultra-fast quantum computers. The light could be made to carry so-called quantum information, which involves particles that can exist in many places or states at once. Computers employing such technology could run through operations vastly faster than existing machines.
[ See also Electromagnetic spectrum ; Interference ; Photoelectric effect ; Wave motion ]