The interference pattern is obtained using a yellow light of wavelength 0 6000 A , in which 20 equally spaced fringes occupy 2.0 cm

Question

The interference pattern is obtained using a yellow light of wavelength 6000 Å , in which 20 equally spaced fringes occupy 2.0 cm on the screen. On replacing the yellow source by another monochromatic source but making no other changes it is noticed that 30 fringes occupy 2.4 cm on the screen. What is the wavelength of the second source?

(a)  4500 Å

(b)  4800 Å

(c)  5000 Å

(d)  5400 Å 

Answer 1

Correct option  (b) 4800 Å 

Explanation:

If D is separation between sources and screen the fringe width is given as

β = λD/d

⇒ β/λ = D/d

where d = separation between virtual sources. If d and D are fixed, then

β₁/λ ₁ = β₂/λ ₂

In first case 20 fringes occupy 2 cm