Find approximately refractive index of liquid

Question

YDSE is made in liquid. The 10th bright fringe in liquid lies in screen where 6th dark fringe lies in vaccum. The refractive index of the liquid is approximately 

(A) 1.8

(B) 1.54

(C) 1.67

(D) 1.2

Answer 1

Answer: (A) 1.8

Fringe width, β=λD/d. When the apparatus is immersed in a liquid, λ and hence β is reduced μ (refractive index) times.