Question

The distance between two slits on a double is 0.25mm, a red beam with wavelength 750nm is shed on the double slits. A screen is put at 1.5m away from the double slits. What is the vertical distance from 3rd dark fringe on the left to the 2nd bright fringe on the right?

Answer 1

Given the distance between the slits d = 0.25mm, the wavelength of the light = 750nm = 750 x 10-9m, and the distance of the slit and screen is D = 1.5m.

Distance of nth dark fringe from the centre is,

So, the distance of third dark fringe from the centre is,

Distance of nth bright fringe from the centre is,

So, the distance of second dark fringe from the centre is,

Since we need the distance from the third dark fringe on the left and the second bright fringe on the right, the total distance is given by,

So the vertical distance from the third dark fringe on the left and the second bright fringe on the right is 0.02025m.