In Young's double slit experiment, we get `60` fringes in the field of view of monochromatic light of wavelength `4000Å`. If we use monochromatic light of wavelength `6000Å`, then the number of fringes obtained in the same field of view is

Options

A. 60
B. 90
C. 40
D. 1.5

Answer

C

Explanation

`lambda prop `(fringe width)
Higher `'lambda'` means wider fringes
At higher wavelength, same space on the screen is occupied by wider fringe, and hence lesser number of fringe
Also `n_(1)B_(1)=n_(2)B_(2)rArr n_(1) lambda_(1) =n_(2) lambda_(2)`
`rArr (4000/6000) =n/60 rArr n=40 `