What Exactly
Light Is?
Overview
Light makes the world visible to us, and thereby it helps us see the world around us. How would be our lives without light? It is hard to imagine. But have you ever wondered what exactly light is?
The simple answer could be it is a form of energy. No doubt, this is a perfect answer. However, if you deep dive into that question, you encounter many unanswered questions such as what is it made of, how does it propagate, does it require a medium to travel like sound?
Let us explore what exactly light is. Before that, let us quickly review what natural philosophers thought about light.
It Is Particle
From the ancient times natural philosophers have been searching for answers to questions such as what exactly light is and how it propagates.
Ancient Greek philosophers believed that objects produced light, and it travelled to our eyes as rays. Lucretius (99 – 55 BC) thought that light consisted of particles. In the 10th century, Egyptian mathematician Ibn Al-Hayatham (965 – 1040) further developed the particle theory of light and published a book in 1027 AD on optics.
The most significant development of particle theory of light was by Sir Isaac Newton (1642 – 1726) who called the light particles corpuscles. According to him light consisted of stream of particles (corpuscles) and illuminous objects give them out in all directions.
No, It Is Wave
Though Newton’s corpuscular theory could successfully explain some of the light phenomena such as reflection, dispersion, and (partially) refraction, it failed to explain the diffraction phenomenon that occurs when light passes through a very narrow slit.
Around the same time Newton was developing the particle theory of light, the Dutch physicist Christiaan Huygens (1629 – 1695) believed that light was a wave travelling through a perfectly elastic, transparent, massless medium – ether – that pervades the entire universe. It is akin to the ripples that fan out across the still surface of a pond from a dropped stone. According to his wave theory, light waves travel through different materials by the propagation of wavefronts.
Using the idea of wavefronts, Huygens was able to successfully explain all the known phenomena of light including the diffraction. In spite of the success of his theory, the particle theory of Newton was barely questioned for almost 150 years until the advent of the English physicist Thomas Young (1773 – 1829).
Young's
Double Slit Experiment
Proof For Wave Theory
At the beginning of the nineteenth century Thomas Young challenged Newton’s particle theory of light and his work led to the revival of the wave theory of light. His interest in the nature of light made him examine the similarities and differences between light and sound. He realized that light must be a wave, and that wave theory can only explain the phenomenon like diffraction.
He devised an experiment to prove that light must be a wave. His experiments consisted of two narrow slits placed extremely close to each other and a screen as shown in the picture below.
Light from a source goes first through a single slit and then through two slits. What Young saw on the screen convinced him that light must be a wave. If light were particles, Young argued, it should result in two bright slits on the screen. But what he saw on the screen were fringes of bright and dark bands as in the picture above.
With this double-slit experiment, Young convinced the scientific community of his time that light must be a wave. But the critical question reminded. If light were a wave, what is waving? The answer came from the works of James Maxwell who unified electricity, magnetism and light.
Maxwell's Discovery:
It Is Electromagnetic
James Clerk Maxwell was a Cambridge educated mathematical physicist from Scotland. He collaborated with Michael Faraday to unravel some of the mysteries of physics. In that process, they unified, what then considered to be, three different phenomena of physics – electricity, magnetism, and light. Having been inspired by Michael Faraday’s lines of force to explain magnetic induction, Maxwell decided to develop a mathematical theory in support of Faraday’s ideas.
After 18 years of persistent work, from 1855 to 1873, Maxwell published the book “Treatise on Electricity and Magnetism”. He could explain all the observed phenomena of electricity and magnetism mathematically using a set of twenty equations. Oliver Heaviside of London, in 1885 using vector algebra, converted Maxwell’s twenty equations into four simple equations. These four equations are now known as Maxwell’s electromagnetic equations which explain diverse range of phenomena.
Maxwell’s theory not only predicted the existence of electromagnetic waves, but it also concluded that light must be electromagnetic in nature.
Later, Heinrich Hertz of Germany, in 1888, successfully produced and detected electromagnetic waves in free space. And, ironically, during these experiments, he also discovered the photo electric effect, which could only be explained, as Einstein did, if light were a particle.
Thanks to Maxwell, now we know what exactly light is. It is an electromagnetic wave – consisting of electric and magnetic fields vibrating perpendicular to each other and also perpendicular to the direction of propagation of light.
Because light is vibrating electric and magnetic fields, it does NOT require any medium to propagate. There is no need for all pervading ether, with strange properties, as Huygens proposed.
The search for what exactly light is did not end here. The final twist came from Planck and Einstein.
Blackbody Radiation Puzzle
Led To Quantum Theory
In 1887, the German Government founded the Physikalishch-Technische Reichsanstalt (PTR) in Berlin. The need to make a better light bulb was the driving force behind the PTR’s blackbody research program in the 1890s. This research ultimately resulted in the discovery of quantum nature of light and opening a new field in physics – Quantum Mechanics.
German physicists like Gustav Kirchhoff, Wilhelm Wien, Lummer built several equipment to study the problem of blackbody radiation. They carefully studied the relationship between a body’s temperature, wavelength of the light it emits and the light intensity. And they plotted their experimental data in a graph. Their graph was similar to the one below.
The graph illustrates the relationship between the temperature of a body and the emitted spectrum of radiation and the intensity.
At relatively low temperatures, the emitted radiation is mostly in the infra-red region. As the temperature of a body increases, the maximum intensity shifts to shorter wavelengths, successively resulting in red, orange, yellow and finally blueish white light.
The problem with this graph was no one could fully account for the basis of a mathematical formula that would produce the hill-like shape of these graphs.
Planck/Einstein's Quanta
Max Planck, a professor of theoretical physics at Berlin University, was puzzled by their findings. In 1900, in order to explain the above experimental results, as a desperate attempt, Planck heuristically arrived at an equation. The derivation of his equation was based on classical Boltzmann’s statistics.
He reluctantly realized that he could explain the blackbody radiation only by introducing a tiny constant and thereby assuming that the light emission or absorption happened in discrete chunks (quanta.) That constant ‘h’ that he introduced in his equation is Planck constant which is now known as one of the fundamental constants of nature.
Though the equation perfectly matched with experimental data, Planck himself was not happy with his assumption of discrete nature of light to explain blackbody radiation results.
Then came the young Albert Einstein. He showed that light was, indeed, not only a wave but also a particle by explaining the photoelectric effect.
Photoelectric Equation
In the year 1905, 26 years old Albert Einstein published four groundbreaking papers. One of them was on the photoelectric effect. Making use of Planck’s idea of quanta, Einstein was able to show that light was not only a wave but also behaved like particles. Now the light particle is known as photon.
He discovered the above equation to explain the photoelectric phenomenon. For that discovery he was awarded the 1921 Nobel prize in physics.
Conclusion
Now, can we say that we have a definite answer to the question of what exactly light is in regard to wave or particle nature? Here is what Niels Bohr said:
“To ask whether light is either a wave or a particle is meaningless. In quantum mechanics, there is no way of knowing what light ‘really is.’ The only question worth asking is: Does the light ‘behave’ like a particle or a wave? The answer is that sometimes it behaves like a particle and at others like a wave, depending on the choice of experiment.”
This is the dual nature of light.
However, one thing is certain. Light is electromagnetic in nature; meaning it is the propagating vibrations of electric and magnetic fields. That is the reason it does not require any medium to propagate.
You may wonder now, if light waves can behave like particles, what about particles like electrons? Do they behave like waves? Yes, they do. We will examine this question in our next topic on Wave-Particle Duality.
References
- This Biggest Ideas in the Universe – Quanta and Fields: By Sean Carroll
- Quantum by Manjit Kumar
- Einstein by Walter Issacson
- https://youtu.be/2WPA1L9uJqo
- https://www.britannica.com/science/light