Question

In a youngs double slit experiment wave length of the light used is 5000A° and the distance between the slits is 2mm distance of the screen from the slits is 1m find the fringe width and also calculate the distance of 7th dark fringe from central bright fringe?​

Answer 1

Given,

wavelength of light used, λ = 5000A° = 5000 x 10⁻¹⁰ = 5 x 10⁻⁷

distance between the slits, d = 2 mm = 2 x 10⁻³ m

distance of the screen from the slits, D = 1 m

Hence fringe width is given by,

X = λD/d

=> X = (5 x 10⁻⁷ x 1)/(2 x 10⁻³)

=> X = 2.5 x 10⁻⁴ = 0.25 x 10⁻³ = 0.25 mm

Hence the fringe width will be 0.25 mm

Distance of the dark fringes is given by,

xₙ = (2n-1)λD/2d

So the distance of the 7th dark fringe is given by,

x₇ = (14-1) x 0.25/2 mm                    (as λD/d = 0.25 mm)

=> x₇ = 6.5 x 0.25 = 1.625 mm

Hence the distance of 7th dark fringe from central bright fringe will be  1.625 mm