The line-width of a bright fringe is sometimes defined as the separation between the points on the two sides of the central line where the intensity falls to half the maximum. Find the line-width of a bright fringe in a Young’s double slit experiment in terms of λ. d and D where the symbols have their usual meanings.
Given, separation between the slits 
The distance between the screen and the slit, 
Wavelength of light, 
.
Let the maximum intensity be 
and the intensity at y be 
.
Given,
therefore, path difference, 
But the fringe extends to both sides of the maximum. Hence, the line width is
1