In this blog post, we will explore the reasons why light is both a particle and a wave, from ancient philosophy to modern quantum mechanics.
Can a man be a woman at the same time? Or can paper be plastic at the same time? Generally speaking, this is impossible. Men and women, paper and plastic are completely different and have opposite properties. Similarly, waves and particles also have opposite properties. A particle is generally defined as a basic unit of mechanical motion modeled as located at a single point, existing at a specific time and place. Therefore, the motion of particles can be accurately described by their position and velocity. On the other hand, waves refer to the transfer of energy from one place to another through vibration, and unlike particles, they do not exist in a specific place at a specific time, so it is impossible to accurately describe the motion of waves. However, there is something unique in the world that has these two opposite properties at the same time, and that is “light.” Light is both a particle and a wave. This is like answering the question, “Is that box made of paper or plastic?” with “It’s made of paper, but it’s also plastic.” This duality of light is not easy to understand with common sense. Therefore, it took the research and efforts of numerous scientists to first conceive of this duality and explain it logically in a convincing manner, and a heated debate over the nature of light continued among them for a long time.
The study of the nature of light began with ancient Greek natural philosophers who wondered, “How do objects appear to the eye?” Euclid argued that light is a wave based on his observation that when light hits a mirror, the angle of incidence is equal to the angle of reflection. Aristotle also argued that light is a wave based on his theory of the four elements, which stated that all matter is composed of fire, air, water, and earth. However, even in ancient Greece, there were scientists who thought of light as particles. Pythagoras claimed that objects emit particles similar to themselves, and that we perceive them visually when they strike our eyes. A hundred years later, Empedocles explained that what is emitted from the eyes collides with objects and is perceived visually. Both explanations considered light to be particles rather than waves. Thus, the wave and particle nature of light has been discussed since ancient Greece. However, at that time, neither argument was particularly convincing, and there was insufficient experimental evidence and scientific knowledge to spark heated debate among scientists, so they could only offer different explanations and speculations.
The debate over the nature of light began in earnest in the late 17th and early 18th centuries, when modern science began to take shape. Newton, who was the highest authority in the scientific community at the time, published Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light, and for a while, the particle theory became the accepted theory in the scientific community. In Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light, Newton described light as “small objects emitted from matter” and argued that there are light particles of different sizes corresponding to each color. In other words, the largest light particles produce the highest frequency, which is red light, and the smallest particles produce the lowest frequency, which is blue light. Newton also assumed the existence of ether as the medium of light and defined light as “particles traveling through space filled with ether,” explaining various optical phenomena based on this assumption. However, Huygens, a scientist who was active at the same time as Newton, argued that light was a wave based on this ether. Unlike Newton, he defined light as “waves propagating through ether” and successfully explained the refraction and straight-line propagation of light, which could not be explained by Newton’s particle theory or the existing wave theory, through “Huygens’ principle,” named after himself. However, due to the extremely short wavelength of light, it was impossible to observe important wave characteristics such as diffraction and interference through experiments. Therefore, Newton’s particle theory, backed by his reputation, took hold instead of the wave theory and was accepted as the standard theory for over a century.
The wave theory, which had been overshadowed by Newton’s authority, was revived in the 19th century by Young’s “double-slit experiment.” In this experiment, light was passed through a double slit, and the light appearing on a screen beyond the slit was observed. In this experiment, Young observed diffraction and interference, which are important properties of waves.
If light were particles, two bright lines should have been observed on the screen because there were two slits. However, what was actually observed were multiple bright lines, and this result could be explained precisely by the diffraction and interference of waves. Therefore, Young’s double-slit experiment gave strength to the wave theory. The rise of the wave theory did not stop there.
In 1867, Maxwell in England discovered that electricity and magnetism are closely related and that the interaction between electric and magnetic fields propagates in the form of waves called electromagnetic waves. He calculated the speed of electromagnetic waves in a vacuum, and the result was exactly the same as the speed of light. This mathematically proved that light is an electromagnetic wave, that is, a wave propagated by the vibration of electric and magnetic fields.
Furthermore, Hertz later proved this experimentally, and the wave theory seemed to be almost established. However, new phenomena were observed that could not be explained by the wave theory, which seemed to accurately explain the nature of light. In 1902, Lenard observed the photoelectric effect and discovered a new fact.
The photoelectric effect is a phenomenon in which photons are emitted when light is shone on a substance. If light is a wave, the energy of the emitted photons should depend on the intensity of the light. However, Lenard’s observations showed that the energy of the photons depended on the frequency of the light, not its intensity, which directly contradicted the wave theory already confirmed by Maxwell and Hertz.
This threw the scientific community into confusion. Lenard’s observations clearly proved that the photoelectric effect demonstrated the particle nature of light, but the wave nature of light, which had been proven by earlier scientists such as Young, Maxwell, and Hertz, could not be denied. It became impossible to say whether light was a particle or a wave. The person who solved this dilemma was the great Albert Einstein. Einstein published three papers in 1905, and the answer was found in the “photon hypothesis” introduced in one of them. Einstein broke away from the framework that light has only one property, either particles or waves, and introduced the concept of “photons,” which have both particle and wave properties. He viewed light as a stream of photons. This idea successfully explained the contradictions that had been revealed until then, and Einstein was awarded the Nobel Prize in Physics in 1921 for his photoelectric effect hypothesis.
Thus, it was revealed that light cannot be explained as either a particle or a wave, but has a dual nature, being both a wave and a particle. This unfamiliar but revolutionary duality is another feature of the microscopic world and forms the basis of quantum mechanics, which plays a very important role in modern physics. Today, quantum mechanics plays an indispensable role in modern life, such as in semiconductors and quantum computers, and scientists are still working to solve new problems in this new field of study.
