DIFFRACTION

When light waves pass through an aperture/obstacle, it bends and spread to some extent that is not directly exposed by light sources. This phenomenon is known as diffraction of light. To explain diffraction, Huygens considered each point a source of secondary wavelets from which new waves emerge and move forward. This understanding had far reaching consequences later on.

Young's double slit experiment

The geometrical arrangement is shown below:

Here, in the experiment a light source is used and S (refer to the fig. on right)is a slit through which light reaches the screen. Before the screen, there are two other slits (S₁ and S₂) that are placed symmetrically with each other with very less distance separating them. Then there is a screen at a large distance.

When monochromatic light wave is allowed to fall on slit S, alternate dark and light bands are observed at the screen with equal width. These bands are called fringes and the whole pattern is called interference pattern.

Explanation

Cylindrical wave-front starts from slit S. Now, according to Huygens ' Principle S₁ and S₂ act as source of secondary wavelets and new waves emerge from them and travel in all possible directions. These waves meet each other at several points in between the screen and source. If the trough of one wave crosses the crest of another, then they are out of phase with each other otherwise they are in same phase.

The bending of light around the corners of the obstacle is termed as diffraction. To see diffraction, it is necessary to have the dimension of the aperture nearly equal to the wavelength of light.

In a homogeneous medium, light travels in straight lines but here it not only travels in straight lines but also bend around the corners of the obstacle.

There are two types of diffraction: (i) Fresnel's diffraction (ii) Fraunhofer's diffraction

In Fresnel's diffraction, the source and the screen are separated by finite distances. Wave-front is either spherical or cylindrical.

Whereas, in fraunhofer's diffraction, they are infinitely separated. Wave-front is necessarily plane. Lens is required to observe Fraunhofer diffraction.

The general condition for the formation of n^th maxima is:

The grating element is determined by no. of rulings per cm on the grating. If this number is N

Then, N (e+d)= 1cm

(e+d)=1/N

DIFFERENCE BETWEEN INTERFERENCE AND DIFFRACTION