Nature and Sources of Light

Optics is a branch of physics which deals with study of phenomena like reflection, refraction, dispersion, interference, diffraction etc. The subject of optics has been divided into ray optics and wave optics. The branch of optics that deals with the production, emission and propagation of light, its nature and the study of the phenomenon of interference, diffraction and polarization is called wave optics.

Nature and Sources of Light

Scientists have made discussion on true nature of light and have postulated various theories. These are discussed below:

1. Newton’s Corpuscular Theory
   Newton put forward this theory that consist of tiny particles, called corpuscles, which are shot out by luminous objects such as an electric lamp, candle, etc. According to Newton, these particles travel in straight lines and bounce off or pass into an object striking off or pass into an object on striking it. According to Newton, these particles travel in straight lines and bounce off or pass into an object striking on it. The speed of light is increased in denser medium and this resulted in a change in direction as corpuscles are attracted by the material of denser medium. However, speed of light decreases in denser medium. This theory cannot explain interference, diffraction and polarization of light. So, this theory is incomplete and inaccurate.
2. Huygens’ Wave Theory
   According to this theory, light propagates from the source in the form of a wave. For the propagation of wave a medium is necessary. So, it was assumed that space is filled with medium called ether, which has the property of both elasticity and inertia. This theory says that light wave is longitudinal but actually light wave is transverse.
3. Electromagnetic Theory
   Theoretically light gets propagated in the form of electromagnetic waves, consisting of electric and magnetic fields mutually perpendicular as well as transverse to the direction of propagation of light. The electromagnetic waves propagate in free space with the velocity of light. Hertz demonstrated experimentally that electromagnetic waves propagate with velocity equal to that of light.
4. Quantum Theory
   in 1905 AD Einstein proposed a new theory of light called a quantum theory, in order to explain the photoelectric effect. According to this theory, light is transmitted as tiny packets of energy called photons and energy of each photon is given by
   $$ E = hf $$
   where f is the frequency of light and h is Planck’s constant.

5. Duel Nature of Light
   Light can exist in both particle and wave form. Using quantum theory, we can explain photoelectric effect. But this theory could not explain interference, diffraction and polarization. These phenomena can be explained using wave theory but wave theory could not explain photoelectric effect.

Wave front

According to wave theory, a source of light sends out disturbance or waves in all directions. A wave front as the locus of all adjacent points at which the phase of vibrations of a physical quantity associated with the wave is the same.

Types of Wave front

Depending on the nature of the source of light, there are three types of wave fronts:

1. Spherical Wave front
   It is produced by the point source of light. This is all because such points which are equidistant from the point source will lie on a sphere. A spherical wave front is shown in figure.
2. Cylindrical Wave front
   When the source of light is linear in shape (e.g a slit), all points equidistant from the linear source lie on the surface of a cylinder. Such a wave front is called a cylindrical wave front as shown in the figure.
3. Plane Wave front
   A small portion of cylindrical wave front or spherical wave front originating from a distant source will appear a plane and hence termed as plane wave front.
   
   

   

References

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.