Coherent light with wavelength 600nm passes through two very narrow slits and the interference pattern is observed on a screen 3.00 m from the slits. The first order bright fringe is at 4.94 mm from the centre of the central bright fringe. For what wavelength of light will the first order dark fringe be observed at this same point on the screen?

Options

A. 600 nm
B. 1200 nm
C. 300 nm
D. 900 nm

Answer

b

Explanation

`(lambda_(1) D)/(d) = 4.84 mm`
Let required wavelength be `lambda_(2)`. Then according to give information,
`(lambda_(2) D)/(2d) = 4.84 mm`
From (i) and (ii),
`(lambda_(1))/((l_(2))/(2)) = 1 implies lambda_(2) = 1200` nm