The bandwidth (β) is defined as the distance between any two consecutive bright or dark fringes.

This is the most commonly used method for producing two coherent sources. If two points are chosen on the wavefront by using a double slit, the two points will act as coherent sources.

\(\delta =\cfrac { \lambda }{ 2\pi } \times \phi \) (or) \(\phi =\cfrac { 2\pi }{ \lambda } \times \delta \) 
\(\phi \) = Phase difference,
ઠ = path difference,
λ = wavelength.

The phenomenon of addition or superposition of two light waves which produces increase in intensity at some points and decrease in intensity at some other points is called interference of light.

Salient features of corpuscular theory:
(i) A luminous body emits tiny massless perfectly elastic particles called corpuscles.
(ii) As the corpuscles are very small, the source does not suffer appreciable loss of mass.
(iii) They are not affected by gravity. so they travel with high speed in a straight line.
(iv) When these corpuscles impinge on the retina of the eye, the vision is produced.
(v) The different size of corpuscles is the reason for different colors.
(vi) The reflection is due to repulsion of the corpuscles by the medium.
(vii) The refraction is due to the attraction of the corpuscles by the medium.
(viii) This theory could not explain the reason why the speed of light is lesser in denser medium than in denser medium and also the phenomena like interference, diffraction and polarisation.