In biprism experiment, 10th dark band is observed at 2.09 mm from the central bright point on the screen with red light of wavelength 6400 A. By how much will fringe width change if blue light of wavelength 4800A is used with the same setting?
In the biprism experiment, the 10th dark band is observed.
The distance between the mth dark band with the central bright band is
Therefore, the distance for the 10th dark band is
Now, when red light is used, we have
Similarly, for blue light, we have
Now, the fringe width is
`therefore X_r=(lambda_rD)/d` .........(3)
From equations (1) and (3), we get
Dividing equations (1) and (2), we get
Now, from equations (2) and (4), we get
Therefore, the change in fringe width when blue light is used instead of red is Xr-Xb=0.22-0.165=0.055mm