Question

Define fringe width. Write the condition for bright fringes and dark fringes in youngs double slit experiment.

Answer 1

The distance between any two consecutive bright or dark fringe is known as fringe width. Symbolically the fringe width is β and β = λD/d.

Bright Fringes 

By the principle of interference, condition for constructive interference is the path difference = nλ  

xd/D = nλ  

Here, n = 0,1,2.....indicate the order of bright fringes  

So, x = (D/d) nλ  

This equation gives the distance of the nth bright fringe from the point O. 

Dark Fringes 

By the principle of interference, condition for destructive interference is the path difference = (2n-1)λ/2  

Here, n = 1,2,3 … indicate the order of the dark fringes.  

So, x = (D/d) [(2n – 1)λ/2]  

This equation gives the distance of the nth dark fringe from the point O. Thus, on the screen alternate dark and bright bands are seen on either side of the central bright band.