Light: Particle or Wave?
The debate has raged for generations amongst the giants of the physics community regarding the nature of Light, namely whether it is a particle or an electromagnetic wave. For centuries, this mysterious and elusive phenomenon left scientists baffled because with each experiment conducted to define its nature, it seemed to change the way it behaved.
In simple terms, light is one of nature’s freaky exceptions, and is considered to be both a wave and a particle. This variability is also one of the fundamental tenets of the theory of Quantum Mechanics. Let’s look at what happened over the years as people came to this important conclusion.
Light is both a particle and a wave. Light has properties of both a particle and an electromagnetic wave but not all the properties of either. It consists of photons that travel in a wave like pattern. — Einstein
So if Light is both a Particle and a Wave, then how is it Possible? How did they Explain it? to answer that questions lets first read the Theories and Discoveries.
The Theories and Discoveries:
Light is also a Particle! — Einstein
The theory of light being a particle completely vanished until the end of the 19th century when Albert Einstein revived it. Now that the dual nature of light as “both a particle and a wave” has been proved, its essential theory was further evolved from electromagnetics into quantum mechanics. Einstein believed light is a particle (photon) and the flow of photons is a wave. The main point of Einstein’s light quantum theory is that light’s energy is related to its oscillation frequency. He maintained that photons have energy equal to “Planck’s constant times oscillation frequency,” and this photon energy is the height of the oscillation frequency while the intensity of light is the quantity of photons. The various properties of light, which is a type of electromagnetic wave, are due to the behavior of extremely small particles called photons that are invisible to the naked eye.
Light is a Particle?
The idea that light may be a particle was first advocated by Sir Isaac Newton, but the idea didn’t catch on particularly well until the 19th century, when Albert Einstein revived the view. He argued that properties such as the reflection and refraction of light could only be explained if light was made up of particles.
Sir Isaac Newton realized that light had frequency-like properties when he used a prism to split sunlight into its component colors. Nevertheless, he thought that light was a particle because the periphery of the shadows it created was extremely sharp and clear.
Waves do not travel in straight lines and cannot exhibit those properties outlined by Newton and Einstein. However, if that’s true, then why was light rejected as a particle? The partial answer is that it did not fulfill or have all the properties that define a particle. A particle is a minute fragment or a quantity of matter with certain properties, such as mass and volume. The smallest unit of light is considered to be a photon, which does not have mass. Also, results of experiments by other researchers during the period between Newton and Einstein showed light having wave-like properties, which made them conclude that light was energy, instead of matter.
The Particle theory:
Pierre Gassendi, an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi’s work at an early age and preferred his view to Descartes’ theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton’s arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.
Newton’s theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century. The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words, it would become what is now called a black hole. Laplace withdrew his suggestion later, after a wave theory of light became firmly established as the model for light (as has been explained, neither a particle or wave theory is fully correct).
The fact that light could be polarized was for the first time qualitatively explained by Newton using the particle theory.
So, Light is an Electromagnetic Wave?
A number of scientists, including Fresnel, Young and Maxwell, are credited with investigating the wave-like properties of light. A wave is a transfer of energy from one point to another without the transfer of material between the two points. Young performed the single-slit experiment, which was instrumental in establishing the wave-like properties of light, such as interference and diffraction. He passed a beam of light through a slit and observed the image it formed on the screen placed behind the slit screen.
If the corpuscular theory of light (light is a particle) proposed by Newton was true, then the pattern on the screen should have been light in the shape and size of the slit. However, the light pattern on the screen was more diffused/ diffracted, which indicated that light has an interference property, just like those exhibited by energy waves. Interference is a phenomenon in which two waves (considered to be linear systems) either have an additive or subtractive effect on each other’s intensity, which make the resultant wave either greater or lower in amplitude.
Light Is a Wave — an Electromagnetic Wave! — Maxell
The next theory was provided by the brilliant Scottish physicist James Clerk Maxwell (1831 to 1879). In 1864, he predicted the existence of electromagnetic waves, the existence of which had not been confirmed before that time, and out of his prediction came the concept of light being a wave, or more specifically, a type of electromagnetic wave. Until that time, the magnetic field produced by magnets and electric currents and the electric field generated between two parallel metal plates connected to a charged capacitor were considered to be unrelated to one another. Maxwell changed this thinking when, in 1861, he presented Maxwell’s equations: four equations for electromagnetic theory that shows magnetic fields and electric fields are inextricably linked. This led to the introduction of the concept of electromagnetic waves other than visible light into light research, which had previously focused only on visible light.
The term electromagnetic wave tends to bring to mind the waves emitted from cellular telephones, but electromagnetic waves are actually waves produced by electricity and magnetism. Electromagnetic waves always occur wherever electricity is flowing or radio waves are flying about. Maxwell’s equations, which clearly revealed the existence of such electromagnetic waves, were announced in 1861, becoming the most fundamental law of electromagnetics. These equations are not easy to understand, but let’s take an in-depth look because they concern the true nature of light.
The Wave theory:
The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young). Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light and explained colour vision in terms of three-coloured receptors in the eye. Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory. In 1816 André-Marie Ampère gave Augustin-Jean Fresnel an idea that the polarization of light can be explained by the wave theory if light were a transverse wave.
So is it a Wave or a Particle?
In addition, Let’s take more time to Read and Understand the Topic!
This discovery is so important — and Nobel Prize worthy — because Einstein suggested for the first time that light is both a wave and a particle.
The Photoelectric Effect
Einstein then proposed that light is actually made up of tiny packets of energy that travel or propagate in a wave-like manner. The particle he conceived was a photon, and he speculated that when electrons within matter collides with photons, the former takes the latter’s energy and flies out. He went on to argue that the higher the oscillation frequency of the photons that strike, the greater the electron energy that will come flying out. This fact is perfectly illustrated through the double-slit experiment.
The phenomenon was fundamentally significant in the development of modern physics because of the puzzling questions it raised about the nature of light — particle versus wavelike behavior — that were finally resolved by Albert Einstein in 1905. The effect remains important for research in areas from materials science to astrophysics, as well as forming the basis for a variety of useful devices.
So you want to Learn about the Photoelectric Effect? That’ll be our next Topic!