Path difference = n*lambda = d*sin(theta)
where n Is some positive real number, d is the inter-slit distance, theta is the angle is subtended by the fringe at the point lying midway the two slits and lambda is the wavelength of light.
For the very last fringe (bright) on the distant screen, the angle theta = 90 degrees. So the above formula becomes n*lambda = d*sin(90) = d*1 = d
Therefore, n*lambda = d which implies n = d/lambda
However, because the number of fringes has to be a whole number, it becomes necessary to introduce a floor function (aka greatest integer function). That is,
floor(n) = [n] = number of fringes on one half of the screen
Hence, total number of fringes = number of fringes on both halves of the screen plus the lone central maxima at the center of the screen
That is, total number of fringes = 2[n]+ 1 = 2[d/lambda]+1
This is the method of calculating total number of fringes based on the conventional analysis of Young’s Double Slit Experiment
Hope it helps
Thanks and regards
Arun
						

							
Dear student 
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