Question

In YDSE ,the silts are separated by0.28 mm and the screen is placed 1.4 m away. The distance between the dark fringe and fourth bright fringe is obtained to be 0.6cm.Determine rhe wavelength of the light used in the experiment.

Answer 1

d = 0.28 mm
D = 1.4 m
Wavelength = λ 
fringe width = w = D λ / d = 5000 λ 

dark fringe forms at x = (1/2) Dλ/d = w/2
4th bright fringe forms at x = 4 Dλ/d =4 w  from the central bright maximum.

The distance between dark fringe and fourth bright fringe = (7/2) w = 0.6 cm
    w = 1.2/7 cm

Wavelength = λ = 342.8 nm