# Quantum Theory of Light     ## Quantum Theory of Light - Detailed Explanation

Many theories were proposed before the actual discovery of the effects of light. Though the light has been in existence since the existence of the sun, the effects of light were not discovered much later. These theories explain the properties of light and how light transmits. Some of the most popular theories on light are discussed below.

Corpuscular Theory: This theory was given in the seventeenth century by Sir Isaac Newton, which states that light emitted by luminous objects consists of tiny particles of matter called corpuscles. This corpuscle when hit the surface, each particle is reflected back. The theory states that the velocity of light changes with the change in density of the medium. This theory could explain three main phenomena of light: the reflection, refraction, and rectilinear propagation of light.

Wave Theory: This theory was discovered by Christian Huygens in the seventeenth century. It states that light is emitted in a series of waves that spread out from a light source in all directions. These waves are not affected by gravity. According to this theory, light waves are mechanical and transverse in nature. This theory successfully explains the phenomena of reflection, refraction, interference, and diffraction phenomena of light.

According to Newton’s theory, light travelling from air to water will increase the speed, while light entering from air to water will decrease the speed. Huygens disagreed with newton’s theory and said that light travelling from air to water will decrease the speed, and vice versa. Huygens’s theory was proved to be correct later on. Around 100 years later, Thomas Young completely disproved the corpuscular theory by showing that light waves can interfere with each other.

Electromagnetic Wave Theory: This theory was discovered in the nineteenth century by James Maxwell, He proposed that light waves do not require any medium for transmission.  Light waves possess both electrical and magnetic properties and can travel through a vacuum. At any instant of time electric and magnetic fields are perpendicular to each other and also perpendicular to the direction of light. The electromagnetic wave is a transverse wave. At every point in the wave at a given point of time, the electric and magnetic field strengths are equal. The velocity of the waves depends on the electric and magnetic properties of the medium.

Quantum Theory: The quantum theory of light was proposed by Einstein, It states that light travels in bundles of energy, and each bundle is known as a photon. Each photon carries a quantity of energy equal to the product of the frequency of vibration of that photon and Planck's constant.

### Wave Theory of Light

Diffraction and interference are some of the behaviours of waves. Maxwell proposed that light is an electromagnetic wave that travels at the speed of light through space. The light frequency is related to its wavelength according to the following relation.

### Particle Behaviour of Light

In the Photoelectric experiment, the electron is emitted by the metal with a particular kinetic energy.

There exists a critical frequency for every metal, lower than which no electrons are emitted. This describes that the kinetic energy equals the light frequency times a constant, known as Planck’s Constant.

$E_{photon} = hν$

### Wave-Particle Duality of Light

The quantum theory of light was given by Einstein, which describes matter, and light consists of minute particles that have properties of waves associated with them. Light consists of particles known as photons and matter are made up of particles called protons, electrons, and neutrons. When all the theories are put together, it can be concluded that light is a particle with wave behaviour. So light is dual in nature.

### Basis of Modern Physics on Quantum Theory of Light

Modern physics is wholly based on quantum theory as it explains the nature and behaviour of matter and energy at the atomic and subatomic levels. Quantum physics and quantum mechanics are terms used to describe the nature and behaviour of matter and energy at that level. Quantum computing, which employs quantum theory to dramatically boost computing capabilities beyond what is conceivable with today's classical computers, has attracted major funding from several countries.

The German Physical Society accepted physicist Max Planck's quantum theory in 1900. Planck wanted to know why, when a glowing body's temperature rises, the colour of its radiation changes from red to orange to blue. He discovered the solution to his query by supposing that energy existed in discrete units, similar to how matter did, rather than just as a steady electromagnetic wave, as had previously been supposed, and was thus quantifiable. Quantum theory began with the existence of these units as its first premise.

To express these discrete units of energy, Planck devised a mathematical equation including a figure, which he dubbed quanta. Planck discovered that at certain discrete temperature levels (precise multiples of a basic minimum value), energy from a luminous body will occupy different sections of the colour spectrum, as explained by the equation.

Planck anticipated that the discovery of quanta would lead to the development of a theory, but their very existence implied a fundamentally new and basic understanding of nature's principles. In 1918, Planck was awarded the Nobel Prize in Physics for his theory, but during the next thirty years, numerous scientists added to the contemporary knowledge of quantum theory.

### Quantum Theory: A Branch of Physics that Has Evolved

While Albert Einstein's theory of relativity was essentially the result of his efforts, the quantum theory was produced over thirty years by a team of experts. Max Planck proposed that the energies of any harmonic oscillator (see the harmonic motion), such as the atoms in a blackbody radiator, are constrained to particular values, each of which is an integral (whole number) multiple of a basic, minimum value, in his explanation of blackbody radiation in 1900.

The energy E of this fundamental quantum is proportional to the oscillator's frequency v, or E=h, where h is a constant, now known as Planck's constant, with a value of 6.626071034 joule-second. Einstein argued in 1905 that radiation is quantized using the same formula, and he utilised this new theory to explain the photoelectric effect. Following Rutherford's discovery of the nuclear atom in 1911, Bohr utilised quantum theory to explain both atomic structure and atomic spectra in 1913, demonstrating the link between the energy levels of electrons and the frequencies of light emitted and absorbed.

The definitive mathematical formulation of quantum theory, quantum mechanics, was produced in the 1920s. In 1924, Louis de Broglie postulated that particles can show wavelike features as well as particle-like properties, as seen in the photoelectric phenomenon and atomic spectra. C. J. Davisson and L. H. Germer confirmed this hypothesis experimentally in 1927 when they observed diffraction of a beam of electrons analogous to diffraction of a beam of light.

Following de Broglie's idea, two distinct quantum mechanics formulations were provided. Erwin Schrödinger's (1926) wave mechanics make use of the wave function, a mathematical entity that is related to the chance of detecting a particle at a given position in space. Werner Heisenberg's matrix mechanics (1925) does not discuss wave functions or other related concepts, yet it has been proven to be mathematically equal to Schrödinger's theory.

This is all about the quantum theory of light and its explanation. It is considered to be the basis of modern physics and the world of theoretical physicists is focusing on it. Understand its concept and find out how it helps to unravel the various mysteries of the universe.

## FAQs on Quantum Theory of Light

1. What are the key points in the Quantum Theory of Light?

The quantum theory states that particles are waves and waves are particles or in other words, with all particles of matter, waves are associated. It states that light acts as both a particle and a wave.   The quantum theory of light tells us about how matter and light act as a particle and it also tells us about how light and matter act as a wave.

2. What is the importance of the Quantum Theory of Light?

The quantum theory of light was the reason why quantum mechanics came into being. Had it not been Einstein who gave the quantum theory of light first as he explained that light is composed of small particles called photons and there are waves associated with this. This also led to the discovery of the photoelectric effect which further led to the discovery of televisions. This theory also became important for other branches of physics.

3. Which theory best explains every characteristic of light?

Planck's Quantum theory explains all the characteristics of light. According to this theory, light energy is released from the source discreetly in the form of energy packets of specific frequencies called photons or quanta. Photons propagate like waves and interact with matter as particles.

4. Define the following terms: Wavelength, Crest, Trough, Amplitude, and Frequency.

1. Wavelength is the distance between two consecutive waves.

2. The crest is the highest point in a wave.

3. The trough is the lowest point in a wave.

4. The amplitude is how high the wave can go.

5. Frequency can be defined as the number of waves in a given period of time.

5. Write some drawbacks in Newton’s Corpuscular theory, Huygens Wave theory, and Electromagnetic wave theory.

Some Drawbacks in Corpuscular Theory was:

• The theory could not explain the phenomena of interference, diffraction, the polarization of light, etc.

• According to this theory, the velocity of light in a denser medium is greater than the velocity of light in a rarer medium but this is proved wrong later.

Drawbacks in Huygens Wave Theory:

• It falls to explain polarization, photoelectric effect, and Compton effect.

• It fails to explain why light has no longitudinal component.

Drawbacks in Electromagnetic Wave Theory:

• The main drawback of this theory is it failed to explain the photoelectric effect and Compton effect.

• The electromagnetic theory also failed to explain black body radiation.

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