Quantum Theory of Light

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Introduction to Quantum Theory of Light

Many theories were proposed before the actual discovery of the effects of light. Though light is in existence since the existence of the sun, the effects of light were not discovered muchlater. 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 that is 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 traveling 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 traveling from air to water will decrease the speed, and vice versa. Huygens’s theory was proved to be correct later on. After 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 one of the behaviors 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.

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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.

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Ephoton = hν

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Wave-Particle Duality of Light

The quantum theory of light was given by Einstein, which describes that 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 behavior. So light is dual in Nature. 

FAQ (Frequently Asked Questions)

1. 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.

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

Wavelength is the distance between two consecutive waves.

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

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

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

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

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

Some Drawbacks in Corpuscular Theory was:

1) The theory could not explain the phenomena of interference, diffraction, and polarization of light, etc.

2) 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:

1) It falls to explain polarization, photoelectric effect, and Compton effect.

2) It fails to explain why light has no longitudinal component.

Drawbacks in Electromagnetic Wave Theory:

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

2.) The electromagnetic theory also failed to explain black body radiation.