Is light a particle or a wave? (2023)

The exact nature of visible light is a mystery that has puzzled people for centuries, and many scientists and philosophers have struggled to answer the following question: is light a particle or a wave?

Greek scientists of the ancient Pythagorean discipline postulated that every visible object emits a constant stream of particles, while Aristotle concluded that light propagates like waves in the ocean. Although these ideas have undergone many modifications and a significant degree of evolution over time, the essence of the dispute raised by the Greek philosophers remains to this day.

Light Theory: Particle or Wave?

One view sees light as wavy, creating energy that traverses space, much like ripples spreading across the surface of a calm pond after being disturbed by a falling rock. The opposite view holds that light consists of a constant stream of particles, like tiny droplets of water, sprayed from the nozzle of a garden hose.

In recent centuries the consensus has wavered, with one view prevailing for a time only to be overturned by evidence from the other. It was not until the first decades of the 20th century that enough convincing evidence was gathered to provide a comprehensive answer, and to everyone's surprise, both theories turned out to be at least partially correct.

By the early 18th century, debates about the nature of light had divided the scientific community into divided factions fighting vigorously for the validity of their pet theories. A group of scientists who subscribed to the wave theory centered their arguments on the discoveries of Dutchman Christiaan Huygens. The opposing camp cited Sir Isaac Newton's experiments with prisms as evidence that light traveled as a shower of particles, each moving in a straight line until they were refracted, absorbed, reflected, diffracted or otherwise perturbed.

Although Newton himself seemed to have some doubts about hiscorpusculartheory about the nature of light, its prestige in the scientific community carried so much weight that its proponents ignored all other evidence during their bitter struggles.

Huygens' theory of lightrefraction, based on the concept of the wave nature of light, held that the speed of light in any substance is inversely proportional to its index of refraction. In other words, Huygens postulated that the more light is "bent" or refracted by a substance, the slower it would travel as it passed through that substance. His followers concluded that if light consisted of a stream of particles, the opposite effect would occur, because light entering a denser medium would be attracted to the molecules in the medium and an increase, rather than a decrease, in intensity and would experience speed.

Although the perfect solution to this argument would be to measure the speed of light in various substances such as air and glass, the devices of the time were not up to the task. Light seemed to travel at the same speed no matter what material it was passing through. It took more than 150 years before the speed of light could be measured with sufficient accuracy to prove Huygens' theory correct.

Despite Sir Isaac Newton's great reputation, many leading scientists in the early 18th century disagreed with his corpuscular theory. Some have argued that if light were made up of particles, when two beams crossed, some of the particles would collide with each other to create a deflection of the light rays. Apparently this is not the case, so they concluded that light does not have to be composed of individual particles.

The wave theory of light

Huygens, against all intuition, had suggested in his 1690 treatisecontract of lightthat light waves travel mediated through spaceether, a weightless mystical substance that exists as an invisible entity in air and space. The search for ether consumed a significant amount of resources in the 19th century before finally burying it. The theory of the ether lasted at least into the late 1800s, as evidenced by the model proposed by Charles Wheatstone, showing that the ether carries light waves by vibrating at an angle perpendicular to the direction of light propagation, and detailed models by James Clerk Maxwell describes the construction of the invisible substance. .

Huygens believed that the ether vibrates in the same direction as light and forms a wave as it carries the light waves. In a later volumeHuygens Prinzip, brilliantly described how each point on a wave can produce its ownwaves, which then add up to form a wavefront. Huygens used this idea to create a detailed theory of the phenomenon of refraction and explain why light rays do not collide with each other when they cross.

When a ray of light travels between two media with different indices of refraction, the ray experiencesrefractionand it changes direction when going from the first medium to the second. To determine whether the light beam consists of waves or particles, a model can be developed for each of them to explain the phenomenon (Figure 3).

According to Huygens' wave theory, a small portion of each angled wavefront should impinge on the second medium before the rest of the front reaches the interface. This portion begins to travel through the second medium while the rest of the wave is still traveling in the first medium, but travels more slowly due to the second medium's higher index of refraction. Since the wavefront is now traveling at two different speeds, it bends towards the second medium and changes the angle of propagation.

In contrast, particle theory has difficulty explaining why light particles must change direction when passing from one medium to another. Proponents of the theory propose that a special force, directed perpendicular to the interface, acts to change the particles' velocity as they enter the second medium. The exact nature of this power has been left to speculation, and no evidence has ever been gathered to prove the theory.

Another excellent comparison of the two theories concerns the differences that occur when light reflects off a smooth specular surface such as a mirror. The wave theory assumes that a light source emits light waves that propagate in all directions. When colliding with a mirror, the waves are reflected according to the angles of incidence, but each wave is bounced back and forth to create an inverted image (Figure 4). The shape of the incoming waves depends strongly on the distance between the light source and the mirror. Light emanating from a nearby source still retains a highly curved spherical wavefront, while light emanating from a distant source propagates further and hits the mirror with nearly flat wavefronts.

The arguments for a particle nature of light are much stronger in relation to the phenomenon of reflection than to refraction. Light emitted from a source near or far reaches the mirror surface as a stream of particles that bounce or reflect off the smooth surface. Because the particles are so small, very many are involved in an expanding beam of light, where they move very close together.

Upon hitting the mirror, the particles bounce off at different points, so their order in the light beam is reversed upon reflection to create an inverted image, as shown in Figure 4. Both particle and wave theories adequately account for reflection from a smooth surface. However, particle theory also suggests that if the surface is very rough, the particles will bounce off at different angles and scatter light. This theory fits very well with experimental observation.

Particles and waves should also behave differently when they hit the edge of an object and cast a shadow (Figure 5). Newton was quick to point this out in his 1704 bookopticsthat "you never know that the light will follow crooked passages or lean towards the shadow". This concept is consistent with particle theory, which proposes that light particles must always travel in straight lines. When particles hit the edge of a barrier, they cast a shadow because particles unblocked by the barrier continue in a straight line and cannot extend past the edge. At the macroscopic level, this observation is almost correct, but does not agree with results from much smaller-scale light diffraction experiments.

When light passes through a small slit, the beam spreads out and becomes wider than expected. This fundamentally important observation lends considerable credence to the wave theory of light. Like ripples in water, light waves striking an object's edge appear to bend around the edge and into its geometric shadow, an area not directly illuminated by the light beam. This behavior is analogous to water waves wrapping around the end of a raft instead of reflecting off it.

Almost a hundred years after Newton and Huygens established their theories, the English physicist Thomas Young conducted an experiment that strongly supported the wave nature of light. Because he believed that light is made up of waves, Young argued that when two waves of light meet, some kind of interaction would occur.

To test this hypothesis, he used a screen with a single narrow slit to produce a coherent beam of light (containing waves propagating in phase) from ordinary sunlight. When the sun's rays hit the slit, they scatter orbowto create a single wavefront. Allowing this front to illuminate a second screen with two closely spaced slits creates two additional coherent light sources that are perfectly synchronized (see Figure 6). Light from each slit traveling to a single point midway between the two slits should arrive at exactly the same velocity.

The resulting waves should reinforce each other to create a much larger wave. However, when considering a point on each side of the center, light from one slit has to travel much further to reach a second point on the opposite side of the center. Light from the slit closest to this second point would arrive before light from the distant slit, so the two waves would be out of phase with each other and could cancel each other out to create darkness.

As he surmised, Young discovered that light waves from the second set of slits meet and overlap as they propagate (or bend). In some cases, the superposition combines the two waves at exactly the same speed. In other cases, the light waves combined with each other slightly or completely out of phase.

Young discovered that when waves meet simultaneously, they move through a process called "constructive influence. Waves that are out of tune cancel each other out, a phenomenon known asdestructiveInterference. Between these two extremes, varying degrees of constructive and destructive interference occur to produce waves with a wide spectrum of amplitudes. Young was able to observe the effects of the interference on a screen placed a fixed distance behind the two slits. After diffraction, the light recombined by interference creates a series of bright and dark lights.stripesover the screen.

Although obviously important, Young's conclusions were not widely accepted at the time, largely due to overwhelming belief in the particle theory of light. In addition to his observations on the interference of light, Young postulated that light of different colors is made up of waves of different lengths, a fundamental concept that is now widely accepted. In contrast, proponents of particle theory envisioned that different colors were derived from particles that had different masses or were traveling at different speeds.

The interference effect is not limited to light. Waves generated on the surface of a pool or pond propagate in all directions and behave identically. If two waves meet at the same time, they add up to form a larger wave through constructive interference. Crashing waves that are out of tune cancel each other out through destructive interference, creating a flat surface on the water.

Further evidence for the wave nature of light was discovered when the behavior of a light beam between crossed polarizers was carefully studied (Figure 7). Polarizing filters have a unique molecular structure that only allows light to pass with a single orientation. In other words, a polarizer can be considered a specialized typeJealousywith small rows of lamellae aligned in only one direction within the polarization material. If a light beam hits a polarizer, only light beams oriented parallel to the direction of polarization can pass through the polarizer. If a second polarizer is placed behind the first and pointed in the same direction, then light passing through the first polarizer will also pass through the second.

However, when the second polarizer is rotated through a small angle, the amount of light that passes through it decreases. If the second polarizer is rotated so that the orientation is perpendicular to that of the first polarizer, then no light passing through the first polarizer will pass through the second. This effect is easily explained by wave theory, but no amount of manipulation of particle theory can explain how the second polarizer blocks light. In fact, particle theory is insufficient to explain interference and diffraction, effects that were later discovered to be manifestations of the same phenomenon.

The effects observed with polarized light were crucial in developing the concept that makes up lightwould you likeWaves with components perpendicular to the direction of propagation. Each of the transverse components must have a specific orientation direction that allows it to pass through or be blocked by a polarizer. Only the waves with a transverse component parallel to the polarization filter pass, all others are blocked.

In the mid-19th century, scientists became increasingly convinced of the wave nature of light, but an overarching question remained: what exactly is light? A breakthrough came when English physicist James Clerk Maxwell discovered that all forms of electromagnetic radiation present a continuous spectrum and travel through a vacuum at the same speed: 186,000 miles per second. Maxwell's discovery effectively sealed the coffin of particle theory, and by the early 20th century it seemed that the fundamental questions of the theory of light and optics had finally been answered.

A major blow to the wave theory of light came behind the scenes in the late 1880s when scientists first discovered that light could, under certain conditions, knock electrons out of the atoms of various metals (Figure 8). Though initially just a strange and inexplicable phenomenon, it was quickly discovered that ultraviolet light can liberate electron atoms in a wide variety of metals to create a positive electrical charge. The German physicist Philipp Lenard was interested in these observations, which he namedphotoelectric effect. Lenard used a prism to separate white light into its color components, then selectively focused each color onto a metal plate to eject electrons.

What Lenard discovered confused and amazed him. For a given wavelength of light (e.g. blue), the electrons produced a constant potential or fixed amount of energy. A decrease or increase in the amount of light resulted in a corresponding increase or decrease in the number of electrons released, but each retained the same energy. In other words, the electrons that escaped from their atomic bonds had energies that depended on the wavelength of the light, not the intensity. This is contrary to what one would expect from wave theory. Lenard also discovered a relationship between wavelength and energy: shorter wavelengths produced electrons with higher amounts of energy.

The basis for a connection between light and atoms was laid in the early 19th century when William Hyde Wollaston discovered that the sun's spectrum is not a continuous band of light but contains hundreds of missing wavelengths. The German physicist Joseph von Fraunhofer mapped more than 500 narrow lines corresponding to the missing wavelengths and assigned letters to the larger gaps. It was later discovered that the gaps were created by the absorption of certain wavelengths by atoms in the Sun's outer shell. These observations were some of the first connections between atoms and light, although the fundamental implications were not understood at the time.

The particle theory of light

In 1905, Albert Einstein postulated that light might have some particle properties, despite overwhelming evidence for a wave nature. In developing his quantum theory, Einstein mathematically proposed that the electrons bound to the atoms in a metal can absorb a certain amount of light (first asQuantum, but then changed to aPhoton) and thus have the energy to escape. He also speculated that if the energy of a photon was inversely proportional to wavelength, shorter wavelengths would produce electrons with higher energies, a hypothesis that emerged from the results of Lenard's research.

Einstein's theory was consolidated in the 1920s by the experiments of American physicist Arthur H. Compton, who showed that photons have momentum, a necessary requirement to support the theory that matter and energy are interchangeable. Around the same time, French scientist Louis-Victor de Broglie proposed that all matter and radiation have properties resembling both a particle and a wave. De Broglie followed Max Planck's example and extrapolated Einstein's famous formula relating mass and energy to include Planck's constant:

E = mc2= hν

From wheremiis the energy of a particle,Metroat the table,Cis the speed of lighthis Planck's constant andNorteis the frequency. De Broglie's work relating a wave's frequency to a particle's energy and mass was instrumental in developing a new field that would eventually be used to explain the wave and particle nature of light.

Duality of Light: Particle and Wave

Quantum mechanics grew out of the research of Einstein, Planck, de Broglie, Neils Bohr, Erwin Schrödinger and others trying to explain how electromagnetic radiation could show what is now calledduality, or behavior of both particles and waves. Sometimes light behaves like a particle and sometimes like a wave.

This complementary or dual role of the theory and behavior of light can be used to describe all of the known properties that have been observed experimentally, from refraction, reflection, interference and diffraction to findings involving polarized light and the photoelectric effect. . Combined, the properties of light work together, allowing us to observe the beauty of the universe.