# Nature of Light

The scientific study of the behavior of light is called optics and it covers :-

• Reflection of light by a mirror or other object,
• Refraction by a lens or prism,
• Diffraction of light as it passes by the edge of an opaque object
• Interference patterns resulting from diffraction and
• polarization of light.

Any successful theory of the nature of light must be able to explain these and other optical phenomena.

Before the starting of the nineteenth century, light was considered as a beam of particles. These particles are either emitted by the object being viewed or emerges from the eyes of the observer.

Let us discuss all the theories proposed by different scientists.

(1). Corpuscles Theory (Particle theory) of light given by Newton in 1660 :-

Newton gave the particle theory of light. According to this theory, particles are emitted from a light source and these particles stimulate the sense of sight upon entering the eye.

Using this theory Newton explained reflection and refraction. Most scientists accepted Newton’s particle theory.

But the corpuscular theory failed to adequately explain the diffraction, interference and polarization of light.

(2). Wave theory of light given by Christian Huygens in 1678 :-

During the time of Newton, in 1678, the Dutch physicist and astronomer Christian Huygens showed that a wave theory of light could also explain reflection and refraction. He argued that light might be some sort of wave motion. According to Huygens :-

Every point that a luminous disturbance meets turns into a source of the spherical wave itself. The sum of the secondary waves, which are the result of the disturbance, determines what form the new wave will take. This theory of light is known as the ‘Huygens‘ Principle‘.

(3). Demonstration of wave nature of light by Thomas young :-

Newton was not able to explain the phenomenon refraction, using his Corpuscles theory so he invoked an inexplicable force that changes the velocity of light when it goes from one medium to another. He was also not able to explain the phenomenon such as diffraction, interference and polarization of light.

Newton was also curious and puzzled by colored fringes in soap films, but stuck to the corpuscular theory despite its difficulties.

In 1800 Thomas Young noticed these problems and in 1801, he presented a serious challenge to Newton’s corpuscles theory on nature of light by giving his lecture at The Royal Institution in London. He said that light is a kind of wave vibrating in some sort of medium called ether.

In May of 1801, Young came up with an idea of slits experiment to demonstrate the interference of light waves. In November 1801 Young presented his paper “On the theory of light and color” to The Royal Society in which he experimentally explained the interference of light using a water tank to show interference pattern in water.

In 1807 he showed that when light coming from a point light source is shined onto two pinholes, interference fringes can be observed on a screen at appropriate distance away (Young’s Double Slit Experiment) and advocated his theory that light behaves like a wave.

(4). Prediction of Electromagnetic waves by James clerk Maxwell in 1864 by giving a set of equations (called “Jewel of Physics”) :-

In 1873, James clerk Maxwell suggested the electromagnetic theory of light. According to this theory, light consists of high frequency electric and magnetic fields in mutually perpendicular directions, and both being perpendicular to the direction of propagation of light wave.

Heinrich Hertz provided experimental confirmation of Maxwell’s theory in 1887 by producing and detecting electromagnetic waves. He showed that these electromagnetic waves possessed all the properties of light waves.

(5). Plank quantum theory explained by Einstein through photo electric effect in 1905 :-

In 1905, Albert Einstein revived the old corpuscular theory using Plank’s Quantum Hypothesis and through his photoelectric effect experiment showed that light consists of discrete energy packets, called photons.  The energy of each photon is given by :-

$\displaystyle E=h\nu =h\frac{c}{\lambda }$

where the constant of proportionality h = 6.63 × 10-34 J s is Planck’s constant.

In view of these developments, light must be regarded as having a dual nature. Light exhibits the characteristics of a wave in some situations and the characteristics of a particle in other situations.