The Nature of Light: _{Particle and wave theories}

Early theories

For as long as the human imagination has sought to make meaning of the world, we have recognized light as essential to our existence. Whether to a prehistoric child warming herself by the light of a fire in a cave, or to a modern child afraid to go to sleep without the lights on, light has always given comfort and reassurance.

The earliest documented theories of light came from the ancient Greeks. Aristotle believed that light was some kind of disturbance in the air, one of his four "elements" that composed matter. Centuries later, Lucretius, who, like Democritus before him, believed that matter consisted of indivisible "atoms," thought that light must be a particle given off by the sun. In the tenth century CE, the Middle Eastern mathematician Alhazen developed a theory that all objects radiate their own light. Alhazen's theory was contrary to earlier theories proposing that we could see because our eyes emitted light to illuminate the objects around us.

In the seventeenth century, two distinct models emerged from France to explain the phenomenon of light. The French philosopher and mathematician Rene Descartes believed that an invisible substance, which he called the plenum, permeated the universe. Much like Aristotle, he believed that light was a disturbance that traveled through the plenum, like a wave that travels through water. Pierre Gassendi, a contemporary of Descartes, challenged this theory, asserting that light was made up of discrete particles.

Particles vs. waves

While this controversy developed between rival French philosophers, two of the leading English scientists of the seventeenth century took up the particles-versus-waves battle. Isaac Newton, after seriously considering both models, ultimately decided that light was made up of particles (though he called them corpuscles). Robert Hooke, already a rival of Newton's and the scientist who would identify and name the cell in 1655, was a proponent of the wave theory (see our Discovery and Structure of Cells module). Unlike many before them, these two scientists based their theories on observations of light's behaviors: reflection and refraction. Reflection, as from a mirror, was a well-known occurrence, but refraction, the now familiar phenomenon by which an object partially submerged in water appears to be "broken," was not well understood at the time (Figure 1).

Proponents of the particle theory of light pointed to reflection as evidence that light consists of individual particles that bounce off of objects, much like billiard balls. Newton believed that refraction could be explained by his laws of motion, with particles of light as the objects in motion. As light particles approached the boundary between two materials of different densities, such as air and water, the increased gravitational force of the denser material would cause the particles to change direction, Newton believed (see our Density module).

Newton's particle theory was also based partly on his observations of how the wave phenomenon diffraction related to sound. He understood that sound traveled through the air in waves, meaning sound could travel around corners and obstacles; thus, a person in another room can be heard through a doorway. Since light was unable to bend around corners or obstacles, Newton believed that light could not diffract. He therefore supposed light was not a wave.

Hooke and others – most notably the Dutch scientist Christian Huygens – believed that refraction occurred because light waves slowed down as they entered a denser medium such as water and changed their direction as a result. These wave theorists believed, like Descartes, that light must travel through some material that permeates space. Huygens dubbed this medium the aether.

Because of Newton's fame and reputation, many scientists of the seventeenth and eighteenth centuries subscribed to the view that light was a particle. The wave theory of light, however, would receive a major boost at the beginning of the nineteenth century from an English scientist named Thomas Young.

The waves have it

On November 24, 1803, Thomas Young stood before the Royal Society of London to present the results of a groundbreaking experiment. Young had devised a simple scheme to see if light demonstrated a behavior particular to waves: interference. To understand this concept, imagine two waves traveling toward each other on a string, as shown in Figure 2:

When the waves reach the same part of the string at the same time, as shown in the middle diagram, they will add together and create one wave with double the amplitude (height) of the original waves. This adding together of waves is known as constructive interference because the waves combine to construct a new, bigger wave.

Another possible scenario is shown in Figure 3:

Here, the two waves approaching each other have equal and opposite amplitudes. When they pass each other (middle diagram), they completely cancel each other out. This canceling effect is known as destructive interference because the waves temporarily disappear as they pass.

Beam splitting: Young's "Double Slit" experiment

Thomas Young recognized that if light behaved like a wave, it would be possible to create patterns of constructive and destructive interference using light. In 1801 he devised an experiment that would force two beams of light to travel different distances before interfering with each other when they reached a screen. To accomplish this, Young set up a mirror to direct a thin beam of sunlight into a darkened room (and an assistant to make sure the mirror aimed the sun's light properly!). Young split the beam in two by placing a very thin card edgewise in the beam.

When the two beams of light shone on a screen, Young observed a very interesting pattern of light and dark "fringes" where the two beams interfered with each other constructively and destructively (Figure 4). Bright fringes appeared where the intensity of the light hitting the screen was highest, and dark fringes appeared where the intensity was zero. Where the two beams of light were exactly "in phase" (see Figure 5), they interfered constructively and created light that was brighter than either beam by itself. Where the beams of light were exactly "out of phase," they interfered destructively to produce a dark spot where the total light intensity was zero.

To understand the pattern of fringes in Young's experiment, let's examine the movement of two waves in more detail. Imagine starting with two waves that are perfectly in phase, as shown in Figure 6:

If one wave travels a greater distance than the other, the peaks and troughs of the waves will become offset from one another and they may be out of phase when they reach their destination, as shown in Figure 7.

If the difference in distance traveled by the two waves is even greater, they will reach a point where the peak of one wave aligns with the trough of the other. Finally, if the wave that travels farther follows a path that is exactly one wavelength longer than the path the other wave follows (or two or three or any integer multiple longer), then their peaks will again align and they will arrive at their destination in phase, as shown in Figure 8.

Young realized that the bright spots on his screen occurred where the difference in the length of the path traveled by the beams of light was an integer multiple of the wavelength of the light. The waves that met at this spot were perfectly in phase and had formed a bright spot because the peaks and troughs aligned with each other.

At the spots where there was no light at all, the difference in path lengths was a multiple of exactly one half-wavelength, so the two waves were completely out of phase and interfered destructively, as seen in Figure 9.

Through this experiment (often called Young's "Double Slit" experiment, and voted by The New York Times in 2002 as science's fifth most beautiful experiment), Young demonstrated with certainty the wave-like nature of light. His experiment answered Newton's charge that light could not bend around corners or obstacles because, when it bent around the edge of the card, it had.

Physicists now know that waves will go around obstacles – a process referred to as diffraction – but only if the size of the obstacle is comparable to the size, or wavelength, of the wave. The card that Young used in his apparatus was very thin – only about as thick as the wavelength of the light he was using it to divide, so the light did, indeed, bend around the card.

Light theory in the 19th century and beyond

In the face of this compelling evidence, nineteenth-century scientists had to concede that light was a wave. This happened slowly, though, hampered by Newton's reputation and the legacy of his corpuscular theory. Yet, once it did take root, the idea of light as a wave paved the way for the nineteenth-century Scottish physicist James Clerk Maxwell to devise an elegant description of light as a wave, which unified two rapidly developing concepts of physics into one complete theory. It was this description that set the stage for a discovery that would arise 100 years later, when a young German-born patent clerk by the name of Albert Einstein would show that the conception of light as a wave was not entirely correct and thereby revolutionize scientific thinking of the twentieth century.