A short introduction to Optics and the Nature of Light

INTRODUCTION

LIGHT is a form of energy which causes the sensation of vision. Optics is the branch of Physics which studies the nature of light and the various phenomena associated with it.

Nature of Light

The nature of light was not at all thought until Newton attempted on it.

Newton’s Corpuscular Theory

According to Sir Isaac Newton light consists of extremely small spherical perfectly elastic particles called corpuscles which are emanating from a source with tremendous speed and whose impact on the retina causes the sensation of vision. This theory could satisfactorily explain the rectilinear propagation and reflection but the explanation given to refraction of light was found wrong later. Newton said that the particles of a denser medium attracted the corpuscles towards them and causes the light to bend at the interface between the two media. If this were correct, light could travel with a greater speed in a denser medium as it would be accelerated more due to the force of attraction by the particles of the denser medium.  But when the velocity of light in different media was found experimentally, it was observed that the velocity of light is less in a denser medium. Further, Newton’s theory could not explain color vision, interference (color in soap bubble and thin films), diffraction (bending of light at sharp opaque obstacles) etc .

Huygens’ Wave Theory

The wave theory proposed by Christian Huygens in 1678 could explain all the above phenomena satisfactorily but failed to explain photoelectric effect (the emission of electrons from the surface of alkali metals when light is incident on it).  Further, Huygens considered light as a longitudinal waves, but the phenomenon of polarization suggests that light is transverse in nature.

Electromagnetic Wave Theory

James Clark Maxwell established the Electromagnetic wave theory in 1864 and he calculated the velocity of electromagnetic waves in vacuum to be 3 x 10⁸ m/s which coincided with the value of experimental value of velocity of light in air. This coincidence led the scientists to conclude that light is an electromagnetic wave.

Dual Nature of Light and Quantum Theory

Even the electromagnetic wave theory could not explain photoelectric effect (for which the particle nature of light is to be used where a ‘light particle’ strikes off an electron from a metal surface). So we have to consider that light has a dual nature; ie, it is a particle as well as a wave at the same time.

According to Max Plank’s Quantum Theory, light is an electromagnetic form of energy emitted as packets called quanta (quantum is the singular form) or photons. The photons possess dual nature.