Wave front

Newton's corpuscles theory

1) Every source of light emits large number of tiny particles known as corpuscles in a                medium surrounding the source.  

2) The tiny particles travel with high velocity, perfectly elastic, & having negligible mass.

3) The corpuscles would travel faster in the denser medium than in rarer medium.

4). When the corpuscles hit our retina in the eye, we get a sensation called light.

5). Different colors of light are due to different size of corpuscles.

Corpuscles theory could not explains

1) Reflection & refraction on the surface of liquid.

2) Could not explain about Interference, diffraction & Polarization.

3) Speed of light in denser medium as per corpuscular - theory should increase, but in              practice found to be decrease.

4) The mass of source must decrease after long emission but it never happens.

      therefore corpuscles theory failed.

Wave front- It is the locus of all points of light vibrations which are at the same phase. All points have same energy & the direction of light is perpendicular on wave front.

Depending upon the shape of source of light, wave front is classified into three types-  

a) Spherical wave front- when the source of light is a point source, then the generated wave front is a sphere with center at the source. It has also two types - i) Converging spherical wave front   (ii) Diverging spherical wave front.

b) Cylindrical wave front- When the source of light is linear, all the points equidistant from the source lie on a Cylinder. Therefore the wave front is cylindrical.

c) Plane wave front- When the point source or linear source of light is at very large distance, a small portion of spherical or cylindrical wave front appears to be plane, such a wave front is plane wave front.

E.g. the wave front generated by the sun is spherical but since it is coming from a very large distance therefore the diameter of this wave front  is also very large & the small portion of this wave front is appears to be plane.