Question

Two slits separated by distance 1mm are illuminated with red light wavelength 8.5x10^-7m. The interference fringes are
observed on sereen placed 1m from the slits. The distance between third dark fringe and fifth bright fringe on the same side of centre is equal to
(A) 0.65mm
(B) 0.63mm
(C) 3.25mm
(D) 4.88mm​

Answer 1

Answer:

2.125mm

Explanation:

Given:

Slit separation, d=1mm=1×10  ^−3  m

Wavelength, λ=8.5×10  ^−7 m

Screen distance , D=1m

To find:

Distance between 3  rd  dark fringe and 5 th  bright fringe

We know,  

For distance of dark fringe, y  = (2n−1)λD /2d

And, for distance of bright fringe, y =  nλD /d

y  5  −y 3  =  5 x 8.5 x 10^-7 x 1 / 10^-3 - (2 x 3 - 1) x  8.5 x 10^-7 x 1 / 2 x 10^-3

= 42.5 x 10^ -4 - 21.25 x 10^ -4

= 21.25 x 10^-4 m

Hence the distance between the third dark fringe and fifth bright fringe is 2.125mm