A beam of light consisting of two wavelengths, 650 nm and 520 nm, is used to obtain interference fringes in a Young’s double-slit experiment.

(a) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650 nm.

(b) What is the least distance from the central maximum where the bright fringes due to both the wavelengths coincide?

Distance of nth fringe from central maxima, Xn = nλD/d

Where, λ is wavelength of light

D is distance from slits to screen

d is slit width

(a) We need to find distance of 3rd from central maxima for wavelength 650 nm

n = 3

x3 = 3 × (650 nm) D/d

(b) The distance of nth bright fringe from central maxima for two wavelengths, say λ1 and λ2, are

Xn = nλ1D/d

Yn = nλ2D/d

When bright fringe due to two wavelength coincide, their distance from central maxima is same

i.e., Xn = Yn

n1λ1D/d = n2λ2D/d

n1λ1 = n2λ2

n1/n2 = λ21

= 520 / 650 = 8/10

Therefore, the bright fringe due to two wavelength coincide when n1 is integer multiple of 8 and n2 is integer multiple of 10.

Thus least distance for which they coincide,

S = n1 λ1D/d

= 8 × (650 nm) D/d

[The question is not complete. This question cannot be answered completely without the values of d and D.]