In young's double slit experiment the separation `d` between the slits is `2mm`, the wavelength `lambda` of the light used is `5896 Å` and distance `D` between the screen and slits is `100 cm`. It is found that the angular width of the fringes is `0.20^(@)`. To increases the fringe angular width to `0.21^(@)`(with same `lambda` and `D`) the separtion between the slits needs to be changed to

Options

A. 2.1 mm
B. 1.9 mm
C. 1.8 mm
D. 1.7 mm

Answer

B

Explanation

Angular fringe width = `lambda/d`
`0.20^@ = lambda/(2 mm)" "….(i)`
`0.21^@ = lambda/d" "…..(ii)`
Dividing equations (i) and (ii), we get `(0.20)/(0.21) = d/(2 mm)`
`d = 1.9 mm`