wave optics

Newton’s Corpuscular Theory

v  Newton thought that light is made up of tinny, light and elastic particles called corpuscles which  are emitted by a luminous body.

v  The corpuscles travel with speed equal to the speed of light in all directions in straight lines.

v  The corpuscles carry energy with them. When they strike retina of the eye, they produce sensation of vision.

v  The corpuscles of different colour are of different size.

v The corpuscles theory explains that light carry energy and momentum, propagation of light  in vacuum and law of  Reflection and Refraction.

 Reason Of Failure

v  The corpuscular theory fails to explain interference, diffraction  and polarization.

v  A major prediction of the corpuscular theory is that the speed of light in a denser medium is more than the speed of light in rarer medium. The truth is that the speed of light is lesser in rarer medium . Therefore, the Newton’s corpuscular theory is wrong.

Huygen’s Wave Theory

According to this theory

v Light travels in the form of wave.

v When light waves are emitted from the source, they travel with uniform velocity in homogeneous medium.

v Different colors are due to the different wavelengths of light waves.

v When light waves enter our eyes, we get the sensation of light.

v Light waves are assumed to be longitudinal type of waves.

     For the propagation of light through vacuum, Huygens suggested the existence of imaginary medium called ether. 

Drawbacks

v Huygens assumed the presence of ether everywhere, but experimentally proved that such ether is not present.

v The phenomenon such as photoelectric effect, Compton Effect cannot be explained on the basis of this theory.  

v Huygens wave theory cannot explain the phenomenon of polarization of light, because Huygens‟ assumed that light waves are longitudinal waves. 

Wavefront

A wavelet is the point of disturbance due to propagation of light.

A wavefront is defined as the continuous locus of those particles  of

a medium , which are vibrating in the same phase .

Depending on the shape of the source of light, the wavefront can be

of three types :

1.Spherical Wavefront

2.Cylindrical Wavefront

3.Plane Wavefront  

Spherical Wavefront

A wavefront which is originated from a point source of light at

 a finite distance. 

Cylindrical Wavefront

                                                                  

A wavefront which is originated from a linear source of light at a finite distance.         

Plane Wavefront

                                                     

When the point source or linear source of light is at very large

distance, a small portion of spherical and cylindrical wavefront

appears to be plane , such a wavefront is called a plane wavefront.

Huygens’ Principle of Secondary Wavelets

                                              

1. Every point on the given wavefront (called Primary wavefront) acts as a fresh source of new disturbance, called secondary wavelets, which travel in all directions with the velocity of light in the medium.

2. A surface touching these secondary wavelets, tangentially in the forward direction at any instant gives the new wavefront  at that instant. This is called secondary wavefront. 

https://vardhmanphysics.blogspot.com/2020/06/reflection-on-basis-of-wave-theory.html

https://vardhmanphysics.blogspot.com/2020/06/refraction-on-basis-of-wave-theory.html