Theories on Nature and Propagation of Light, Physics, Grade 12

In order to deal the nature and the way of propagation of light in the medium or space, different theories have been invented. Some of the important theories are described below:

I. Newton’s Corpuscular Theory:-

According to this theory, “Light is made up of very tiny particles called corpuscles, which are shot out by luminous object”.

Some of the important points of this theory are:

1. Light consists of very tiny elastic particles, called corpuscles.
2. Corpuscles are mass-less particles and propagate with the speed of light (3 X 108 m/s).
3. When corpuscles strike on the object, either they are bounced off or passed inside the object.
4. The sensation of vision is due to the entering of corpuscles into our eyes.
5. The different colors of light is due to the different sizes of corpuscles.
6. This theory couldn’t explain the concept of interference, diffraction and polarization of light, due to which this is incomplete and inaccurate theory.

II. Huygen’s Theory:

According to this theory, “Light propagates in the form of longitudinal wave due to which it needs medium for propagation. So, the space including vacuum is filled by new medium called ether”.

Some of the important points regarding this theory are:

1. Light propagates in the form of longitudinal wave, i.e. it doesn’t propagate without presence of any medium.
2. The space including vacuum is filled by the new medium, which is called ether, which is elastic and inertial in nature.
3. The medium ether distributes energy equally in all directions.
4. Huygen, himself, explained the concept of reflection and refraction of light on the basis of this theory.
5. Young explained the interference of light and Fresnel explained diffraction as well as polarization of light on the basis of this theory.

III. Electromagnetic Theory:

According to this theory, “Light consists of both electric and magnetic field which are acting perpendicular to each other such that propagation of light is transverse in nature”. According to this theory, the light consisting both fields propagate with the speed of 3 X 108 m/s.

IV. Quantum Theory:

According to this theory, “Light is emitted or transmitted in the form of very tiny packets of energy called photons.” The energy of each photon is given by; E = hf, where ‘f’ is the frequency of incident photon and ‘h’ is the Planck’s constant whose value is 6.62 X 10-34 Js. Depending on the Quantum theory, Einstein explained photoelectric effect.