Light has the unique property of being both particle and wave. From ancient Greece to modern times, scientists have proposed various theories to explain light’s duality, and eventually Einstein’s photon hypothesis successfully explained it.
Can you be a man and a woman at the same time, or paper and plastic at the same time? In general, it would be impossible. Man and woman, paper and plastic are too different, too diametrically opposed. Similarly, waves and particles have opposite properties. A particle is a fundamental unit of motion in mechanics that is typically modeled to be located at a single point, and exists in a single place at a specific time. The motion of a particle can therefore be described precisely in terms of position and velocity. A wave, on the other hand, is the transfer of energy from one place to another through oscillations, and unlike particles, it is impossible to describe the motion of a wave precisely because it does not exist in one place at any given time. However, there is something unique in the world that has both of these opposite properties at the same time: light. Light is both a particle and a wave. It’s like answering the question, ‘Is the box paper or plastic?’ by saying, ‘It’s both paper and plastic. This duality of light is not easy to understand using common sense. Therefore, it took the research and efforts of many scientists to first come up with this duality, explain it logically, and convince people, and the heated debate over the nature of light lasted for a long time.
The study of the nature of light began with the natural philosophers of ancient Greece, who asked the question: How do objects become visible to the eye? Eukleides argued for a wave theory based on the observation that the angle of incidence and reflection of light in a mirror is the same, and Aristoteles argued for a wave theory based on his four-element theory that all matter is composed of four elements: fire, breath, water, and earth. But even in ancient Greece, there were scientists who thought of light as a particle. Pythagoras argued that objects emit particles that are similar to themselves and that we perceive them visually when they hit our eyes. A hundred years later, Empedocles explained that we perceive what is emitted from the eye as it strikes an object, and both of these explanations conceive of light as particles rather than waves. The wave and particle nature of light has been discussed since ancient Greece. However, at the time, there were not enough experimentally confirmed facts and scientific knowledge to give any of the arguments much weight, or to spark a heated debate among scientists, so different explanations and speculations were offered.
The debate over the nature of light really heated up in the late 17th and early 18th centuries, when modern science began to take hold. Newton, the foremost authority in the scientific community at the time, published his book Optics (1704), which, for a time, led to the acceptance of the particle theory as the orthodoxy of the scientific community. In Optics (1704), Newton described light as a “small object emanating from matter” and argued that there are different sizes of light particles corresponding to each color of light: the largest particles produce the greatest frequency of light, which is red light, and the smallest particles produce the least frequency of light, which is blue light. Newton also postulated the ether as the medium of light and described light as a “particle traveling through the ether-filled space of the universe,” which explains many optical phenomena. However, Huygens, a scientist who was a contemporary of Newton, argued for the wave nature of light based on the ether. Unlike Newton, he defined light as a wave propagating through the ether, and successfully explained light refraction, straightening, and other phenomena that could not be explained by Newton’s particle theory and the existing wave theory through his named Huygens’ principle. However, due to the very short wavelength of light, important wave features such as diffraction and interference were impossible to observe experimentally, so the particle theory, backed by Newton’s reputation, gained ground and was accepted as the orthodoxy for more than a century.
Overshadowed by Newton’s authority, wave theory was revived in the 19th century by Young’s “double slit experiment”. By passing light through a double slit and observing its appearance on a screen beyond the slit, Young observed diffraction and interference, two important properties of waves. If light were a particle in this experiment, we should have seen two bright lines on the screen because there were two slits. But what was actually observed were multiple bright lines, which could be explained precisely by the diffraction and interference of waves. Young’s double-slit experiment thus gave the wave theory a boost.
The rise of wave theory did not stop there. In 1867, the Englishman Maxwell discovered that electricity and magnetism were closely related, and that the interaction of electric and magnetic fields propagated in the form of electromagnetic waves. He calculated the speed of electromagnetic waves in a vacuum, which was exactly the same as the speed of light. It was mathematically proven that light is an electromagnetic wave, a wave propagated by oscillating electric and magnetic fields. And when Hertz later confirmed this experimentally, the wave theory seemed to be all but established.
However, a new phenomenon was observed that could not be explained by the wave theory, which seemed to explain the exact nature of light. In 1902, Lenard observed the photoelectric effect. If light were a wave, the energy of the photoelectrons should depend on the intensity of the light. However, Lennart’s observations showed that the energy of the photoelectrons did not depend on the intensity of the light, but on the frequency of the light, which directly contradicted the wave theory already confirmed by Maxwell and Hertz.
The scientific community was thrown into turmoil. While Lennart’s observations made it clear that the photoelectric effect proved the particle nature of light, the wave nature of light, as demonstrated by Young, Maxwell, Hertz, and others, could not be denied. Light could neither be called a particle nor a wave. It was the great Einstein who solved this dilemma. In 1905, Einstein published three papers, one of which, the Photon Hypothesis, was the answer. Einstein broke away from the idea that light was either a particle or a wave, and introduced the concept of a “photon,” which has both particle and wave properties. He viewed light as a stream of photons. This way of thinking successfully explained the contradictions in previous findings, and the photon hypothesis won Einstein the Nobel Prize in Physics in 1921.
It turned out that light cannot be described as either a particle or a wave, but is both a wave and a particle. This strange but revolutionary duality is another characteristic of the microscopic world, and is the basis of quantum mechanics, a very large part of modern physics. Today, quantum mechanics is an integral part of modern life, including semiconductors and quantum computers, and scientists are still solving new problems with this new discipline.
