Dual Nature of Light and Matter – Wave and Particle

Scientists have been perplexed by the true nature of light since ages. They still are!

However, many theories have been proposed regarding the same, and some of them have even been supported by experiments. In this article, we will discuss about these with an aim to find out what light really is. All this will also be applicable to quantum-sized matter particles.

But before we do so, let’s have some historical context.

Table of Contents

• Dual Nature of Light
• Dual nature of Matter

Dual Nature of Light

Historical Context of the Nature of Light

• As per Newton’s Corpuscular theory, light was considered to be consisting of a stream of corpuscles.
• Then around the middle of the 17th century, the idea of light being a wave was propounded.
• At the start of the 20th century, work of Albert Einstein and some other scientists proved that light do act as if it’s made up of discreet particles.

Now, there is a general consensus among scientists that light has a dual nature – sometimes it behaves as a wave, and sometimes as a stream of particles. That’s just because some of the characteristics of light fit well with the wave nature of light, while others can be explained better if we perceive light as particles (called photons).

So, now let’s study this in more detail. But keep in mind that this is still a work in progress. Scientists still do not understand the nature of light completely.

Wave Nature of Light

• Grimaldi (in around 1663) noted a phenomenon of light called diffraction, which is the bending of light waves around the edges of an object. In 1665, Hooke showed that diffraction could be explained by wave theory of light.
• In 1670, Huygens also proposed the idea that light might be a wave. He showed that wave theory of light could explain the laws of reflection and refraction.
• Around 1827, many experiments were conducted by Young and Fresnel to study the phenomenon of interference of light (e.g. double-slit experiment). They conclusively proved the wave nature of light.
• The next giant leap in wave theory of light came from Maxwell, who theoretically showed that electromagnetic waves are radiated by oscillating electrical circuit. And that the speed of these electromagnetic waves is equal to the speed of light. Perhaps light is an electromagnetic wave. This is called the Maxwell’s electromagnetic theory of light. But this remained only a theory for the next fifteen years.
• In his experiments, Hertz successfully produced short-wavelength electromagnetic waves. He also proved that these waves possessed all the properties of light waves - reflection, refraction, polarization etc. Thus, he was the one who experimentally verified Maxwell’s electromagnetic theory of light. This was a revolutionary find in physics of that era.

By the end of the nineteenth century, all these experiments led the science community to believe that light is indeed just a (electromagnetic) wave. But there was a big problem. While wave theory of light did explain some of the phenomena of light pretty convincingly, it failed to explain many other phenomena. Let’s have a look at both of these sets of phenomena.

The properties of light that are explained better if we perceive light as a wave are:

• Propagation of light
• Reflection
• Refraction
• Diffraction
• Polarization
• Interference (probably the strongest proof of the wave nature of light)

However, wave theory of light cannot explain the following properties of light:

• Photoelectric emission – wave theory could not explain why photoelectric emission is instantaneous, and the concept of threshold frequency.
• Compton’s effect
• Raman effect

Particle Nature of Light

• In 1900, Max Planck proposed Quantum Theory. As per this theory, electromagnetic wave is composed of discreet quantum energy packets (later called photons). Herein, he gave the famous equation to measure the energy of a photon, E = hv, where h is called Planck’s constant and v is the frequency of emitted photon. This theory laid the foundation of quantum mechanics.
• In 1905, Albert Einstein used Quantum Theory proposed by Planck to explain photoelectric effect. As per him, photoelectric emission takes place due to quanta of energy of radiation, which has energy and momentum (and not by continuous absorption of energy). As light quantum has a definite value of energy and momentum, it’s a strong indication that it may be a particle (later named photon). Thus, a new picture of electromagnetic radiation was proposed by him, wherein light was made up of particles, and was not just a wave.
• In 1924, A.H. Compton conducted an experiment on scattering of X-rays from electrons. This further confirmed the particle nature of light.

Noble Prize for Photoelectric Effect

Nobel Prize in Physics was awarded to Einstein for his contribution to theoretical physics and the photoelectric effect.

Nobel Prize in Physics was awarded to Millikan in 1923 for his work on the elementary charge of electricity and on the photoelectric effect.

The properties of light that are explained better if we perceive light as a particle are:

• Photoelectric emission (probably the strongest proof of the particle nature of light)
• Compton’s effect
• Raman effect

That’s because these phenomena involve energy and momentum transfer, i.e. interaction of light with matter.

Characteristics of Photons

Now, let’s have a look at the nature of the particles of light, called photons.

• Each photon has a particular energy and momentum, which may be different for different photons. However, all photons are considered to have the same speed, i.e. the speed of light denoted by c.
• Energy and momentum of a photon is dependent on its frequency and wavelength. In other words, photons having the same frequency and wavelength will have the same energy and momentum.
• Energy of a photon is not dependent on the intensity of radiation. If you increase the intensity of light, only the number of photons per second crossing a given area will increase. All of these photons will have the same energy if their frequency/wavelength are the same.
• The total energy and total momentum of a system of photons and particles is conserved. For example, if we collide a photon with a particle (e.g. photon-electron collision), the total energy and total momentum will remain the same even after the collision. But keep in mind that the number of photons may not remain the same – a collision may lead to absorption of existing photons or creation of new photons.
• Photons are not electrically charged, i.e. they are electrically neutral. We know this because they are not deflected by electric and magnetic fields.

Nowadays, we consider light to be both a wave and a particle. That is, we accept the dual (wave-particle) nature of light.

This is true for electromagnetic radiation in general, and as we will soon see even for matter particles.

Dual nature of Matter

In 1924, French physicist Louis Victor de Broglie introduced a hypothesis as per which all micro subatomic matter particles (e.g. electron, proton, neutron etc.) also behave like waves when in motion. These waves came to be known as matter waves or de-Broglie waves.

This hypothesis was later on experimentally proved by Davisson-Germer experiment. It proved the wave nature of the moving particles.

In 1989, double-slit experiment was conducted using a beam of electrons (instead of light). It experimentally demonstrated the wave nature of matter.

In 1994, scientists saw interference fringes being formed in the double-slit experiment by beams of iodine molecules. Iodine molecules are about a million times more massive than electrons. It proved that even much larger matter particles may showcase wave nature (and not just elementary particles of matter).

The de Broglie hypothesis is one of the breakthroughs that led to the development of modern quantum mechanics.

De Broglie hypothesis has also led to the field of electron optics. Understanding of the wave properties of electrons helped scientists design electron microscopes, which have much higher resolution than optical microscopes.

Note

Nobel Prize in Physics was awarded to Louis Victor de Broglie in 1929 for his discovery of the wave nature of electrons.

Just as was the case with light/radiation, scientists do not completely understand the dual nature of matter – how does it work physically. But nevertheless, mathematically it has been introduced in modern quantum mechanics with remarkable results.

For example, this led Max Born to suggest a probability interpretation to the matter wave amplitude. According to this, the intensity (square of the amplitude) of the matter wave at a point determines the probability density of the particle at that point (Probability density means probability per unit volume). More the intensity of matter wave in a certain region, the higher will be the probability of us finding the particle there, and vice-versa.

Heisenberg’s uncertainty principle

The dual nature of matter fits well with Heisenberg’s uncertainty principle.

As per Heisenberg’s uncertainty principle, we cannot measure both the position and momentum of a quantum-size particle (e.g. electron) at the same time with accuracy. There will always be some uncertainty in either the position (Δx), or momentum (Δp) of the particle, and generally both. Ordinarily, both Δx and Δp are non-zero.