FUNDAMENTAL THEORIES OF LIGHT

Physical optics is a branch of optics which deals with nature of light, mechanism of its production, emission and propagation through space. It deals with the phenomenon of Interference, Diffraction and Polarization of light. From the experimental observations, it is evident that light is form of energy.

There are two methods of transfer of energy.

(i) by stream of particle movement

(ii) by means of wave motion Based on these modes of transfer of energy

(i) Corpuscular theory was proposed by Newton

(ii) Wave theory was proposed by Huygen,

(iii) Electromagnetic theory was proposed by Maxwell and

(iv) Quantum theory was proposed by Max planck

Physics – Newton’s corpuscular theory of light – Science

I Newton Corpuscular Theory (1675):

• The Corpuscular theory of light was proposed by Newton in 1675. According
• to him, light being a form of energy can be transported in the form of particles called
• Corpuscles which are shot by a luminous object giving out light.
1. Luminous body continuously emit tiny, spherical, small, massless, elastic particles
• and he called them corpuscles, in all directions, with a high speed.(Object whether matchstick
• or burning sun).
1. These corpuscles travel in straight lines.
• III. They readily pass through interspaces of matter.
1. Different colours of light rectilinear propagation, reflection of light. But failed to
• explain interference, diffraction, polarization, colours of thin films etc.

II Huygen’s Wave Theory:

• According to this theory, light is a form of energy which advances in the form
• of wave motion. The source of light is a sort of disturbance.(Consisting of simple Harmonic
• disturbance like sound). Huygen assumed that light waves are longitudinal in nature. Now
• just as sound needs a material medium for its propagation, light was also given a medium
• through which it travels. This medium was called “ETHER”. Since light travels in vacuum,
• ether was supposed to fill all space including vacuum. This ether was supposed to be
• massless, colourless, invisible medium having high elasticity and practically no density.
• This theory could explain reflection, refraction, total internal reflection, double
• refraction etc. But failed to explain diffraction, polarization etc. Also, experimentally proved
• the non-existence of ether.

III. Electro-magnetic Theory:

• Maxwell proved that light gets propagated in the form of electromagnetic
• waves. It consists of electric and magnetic fields which are at right angles to each other and
• transverse to the direction of propagation of light. The alternately expanding and collapsing
• 2
• magnetic and electric fields propagate in space with a velocity of 8 1 3 10 ms- ´ . Thus the
• difficulties of „ether‟ were removed.
• This theory satisfactorily explained reflection, refraction, interference, diffraction,
• polarization etc. But it could not explain photoelectric effect, Raman effect, emission of light,
• x-rays, Compton effect etc.
1. Quantum Theory:
• According to this theory, light exhibits particle nature. Planck proposed this
• theory to explain the black body radiation. According to him, light is made up of energy
• packets called photons. Each photon carries an energy E = hg where h is called planck‟s
• constant 34 6.625 10 JS – ´ and g is the frequency of radiation. A photon has no rest mass, it has
• momentum, hence we say that they exhibit particle nature.
• This theory could explain photoelectric effect, Raman effect, Compton effect etc. But
• it could not explain interference, diffraction etc.

Dual Nature of Light:

• Certain observed phenomena such as reflection, refraction, interference, diffraction,
• polarization etc can be explained only if we assume light to be of wave nature. In other
• words, light exhibits wave nature in these experiments.
• Phenomena like emission, photoelectric effect etc can be explained only by assuming
• light to be a particle.
• Hence we conclude that light exhibits both wave nature and particle nature, it exhibits
• dual nature in other words.

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