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How Albert Einstein's Quantum Theory of Light Transformed Physics

Never underestimate the power of light.

1905 is referred to as the "miracle year" by physicists. In that one year, Albert Einstein published four papers that laid the foundations of modern physics.

One of the major breakthroughs proposed by Einstein in 1905 was the quantum theory of light, which posited that light is made up of small particles, known as photons, and these quantum particles have the ability to show wave-like properties.

From laser technology to television screens, there are many inventions that would have never been possible without the knowledge imparted through Einstein's theory. It not only transformed the domain of quantum mechanics but also influenced various other branches of science. 

Principles that led to the quantum theory of light

How Albert Einstein's Quantum Theory of Light Transformed Physics
Source: NASA/Unsplash

Scientists began to explore the various properties of light from as early as the 17th century, in order to understand its behavior, motion, and origin of light and develop ways to use this knowledge. 

Corpuscular theory

Proposed by Sir Isaac Newton, this theory argued against Christiaan Huygens' theory, which stated that light was made of waves, by suggesting that the geometric nature of reflection and refraction of light could only be explained if light were made up of particles. He referred to these particles as corpuscles. Newton proposed that every time light rays strike a surface, corpuscles are reflected back, and that the density of a medium affects the velocity of light. 

Huygens principle and the wave theory of light

How Albert Einstein's Quantum Theory of Light Transformed Physics
Thomas Young, Source: Henry Perronet Briggs/Wikimedia Commons

Contrary to Newton, Dutch Mathematician Christiaan Huygens argued that light is made up of waves that propagate in a perpendicular fashion with respect to the direction of light. He further explained that every point that a luminous disturbance meets turns into a source of the wave itself. A new wave is then determined by the sum of the secondary waves, that result from the disturbance. Huygens' principle was introduced in 1678 to explain the reflection and refraction caused by light rays. 

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Many years later, in 1801, British scientist Thomas Young conducted his 'double-slit experiment', which validated Huygen’s findings on the wave-like behavior of light. 

How Albert Einstein's Quantum Theory of Light Transformed Physics
Source: Stannered/Wikimedia Commons

In Young’s experiment, a beam of light from a single source was split into two beams, and the two beams were then recombined and superimposed onto a screen, resulting in a pattern of light and dark fringes on the screen. Young concluded that the fringes resulted from the fact that when the beams recombined, their peaks and troughs were not in phase. When two peaks coincide they reinforce each other, and a line of light results; when a peak and a trough coincide they cancel each other, and a dark line results.

The formation of the resultant wave or interference pattern by the superimposition of two waves was referred to as interference. 

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The double-slit experiment produced evidence contrary to Newton’s corpuscular theory, and it was the first practical proof of the wave theory of light. Thomas Young mentioned the experiment in Lecture 39 of his famous book A Course of Lectures on Natural Philosophy and the Mechanical Arts. 

In the years that followed, French engineer August Fresnel’s findings on diffraction, the phenomena due to which light spreads when passed through a narrow aperture, also confirmed the relevance of the double-slit experiment.   

Electromagnetism and quantum theory

James Clerk Maxwell formulated the theory that electric and magnetic fields propagate with the speed of light, and concluded that light is an electromagnetic (EM) wave. He also predicted the presence of the numerous EM waves that form the electromagnetic spectrum.

According to Maxwell’s wave theory of light:

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𝜈 = c/λ

where,
𝜈 = frequency
c = speed of light
λ = wavelength

Later, in 1886, Heinrich Hertz built a spark-gas transmitter composed of induction coil and a Leyden jar (a capacitor) to create electromagnetic waves, and a spark gap between two brass spheres to detect them. Using this apparatus, he detected radio waves (which also traveled at the speed of light). Hertz’s experiment proved the existence of EM waves proposed by Maxwell.

In 1900, Max Planck postulated that energy of light is emitted in the form of small packets of energy called quanta; and that the energy of each quanta is directly proportional to its frequency. Planck won the Nobel prize in 1918 for his work, which also set the stage for the development of quantum mechanics. 

Wave-particle duality of light

How Albert Einstein's Quantum Theory of Light Transformed Physics
Source: Pixabay/pexels

The notion that like matter, light also exists in the form of both particle and wave was further explained by Einstein and Louis De Broglie.

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Photoelectric effect

The emission of photoelectrons from a metal surface when light strikes the metal is called the photoelectric effect. The electrons released during this process are called photoelectrons and their emission is influenced by the frequency of the incident beam of light.

How Albert Einstein's Quantum Theory of Light Transformed Physics
Albert Einstein, Source: The Scientific Monthly/Wikimedia Commons

The photoelectric effect was first proposed in 1887 by Heinrich Hertz, who observed the occurrence of electric charge in a cathode ray tube when UV light hit the cathode. In 1897, physicist J.J. Thomson performed a cathode-ray tube experiment, which led to the discovery of electrons. Thomson also proposed the plum pudding model of the atom, in which negatively-charged electrons were embedded like raisins within a positively-charged "plum pudding".

How Albert Einstein's Quantum Theory of Light Transformed Physics
The photoelectric effect. Source: Helen Klus/Flickr

The photoelectric effect was explained in detail by Albert Einstein in 1905, when he proposed that light is made of tiny particles called photons (previously called quanta), with the energy of a photon given as

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E 𝜈
E = h𝜈  (Planck’s equation) or
E = hc/λ

here,
E = energy of a photon
h = Planck’s constant (6.626 × 10-34 m2 kg/s)
𝜈 = frequency of incident light
λ  = wavelength of light
c = speed of light in vacuum

The minimum amount of energy required by an electron to leave the metal surface is referred to as threshold energy, and the minimum value of frequency of light that is sufficient to cause the photoemission of an electron is called threshold frequency. 

Φ = h𝜈th

Φ = hc/λth

here,
Φ = threshold energy
𝜈th = threshold frequency
λth  = threshold wavelength

The photoelectric effect follows the law of conservation of energy which states that energy can neither be created nor be destroyed. The energy of a photon is equal to the sum total of energy required to emit an electron and the kinetic energy of the emitted electron.  

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 h𝜈 = W + E

here,

h = Plank constant
𝜈 = frequency of the incident photon.
W = work function (the minimum photon energy required to liberate an electron from a substance)
E = maximum kinetic energy of ejected electrons (1/2 mv²).

The photoelectric effect not only validated the particle nature of light but also strengthened the possibility of photons acting as a wave (since Einstein’s equation involved both frequency and wavelength). In 1921, Albert Einstein was awarded Nobel Prize in Physics for his exceptional work on the photoelectric effect and the quantum theory of light.

De Broglie Wavelength

De Broglie put forward the idea that light exhibits wave-like properties such as frequency and wavelength, and dual nature is not a special case but the fundamental nature of light energy.

He combined Einstein’s special theory of relativity with Planck’s equation for energy to reveal the wave nature of light in the year 1924.

E = mc2

E = h𝜈 

mc2 = h𝜈

mc = h𝜈/c = p

here,
p = momentum

Now, we know that frequency and wavelength share an inverse relationship, and 

 λ=c/f

p = h/λ

λ = h/p = h/mv

here,
λ = De Broglie wavelength
v = velocity of particle

In his theory, De Broglie explained that λ = h/mv demonstrate the wave nature of particles. He came to the conclusion that if a wave can show particle behavior then a particle (photon) is also able to exhibit the properties of a wave.  

Significance of quantum theory of light in the modern world

How Albert Einstein's Quantum Theory of Light Transformed Physics
Source: Science In HD/Unsplash

More than 100 years have passed since the quantum theory of light got introduced to us, but even today this theory is so relevant that many modern-day discoveries and inventions are found to be based upon its underlying knowledge.

  • Wave optics is a branch of science resulted from the quantum theory of light that deals with diffraction, interference, and polarization of light. Microscopy, an application of wave optics, allows us to see objects which are impossible to view with naked eye. Many important discoveries (related to micro-organisms, body cells, protein structure) would not have been possible without the theories of wave optics.
  • LCD (liquid crystal display) screen used in TVs, calculators, digital clocks, and LCD monitors combines electric field with light energy to produce images. Whereas, optical discs such as CDs and DVDs use laser beam technology to store data digitally. Both of these innovations are based on the principle of interference. 
  • Hologram technology is still under development and it has the potential to bring the virtual world into reality. This exciting next-generation digital technology is based on diffraction and an application of the wave-like property of light. Diffraction is also the core principle behind Spectroscopy, a technique used to detect the elements found in various heavenly bodies.   
  • The quantum theory of light is also used to explain the occurrence of various phenomena such as photolysis, X-ray diffraction, bioluminescence, etc. Recent research shows that a better understanding of the quantum properties of light could lead to further developments in fields such as energy harvesting, quantum information, and cryptography. 

From cosmology to holograms, our understanding of light has changed the world in numerous ways.

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