Question

The absolute refractive indices of glass and water are 4/3 & 3/2, respectively. If speed of light in glass is 2x108 m/s , calculate the speed of light in vacum and water.
Why are we taking Ng as 3/2 and not 4/3

Answer 1

Actually, ng = 3/2 and nw = 4/3.

Absolute Refractive Index of a medium = Speed of light in vacuum/Speed of light in the medium

Speed of light in glass = 2 x 108 m/s
Speed of Light in Vacuum = (3/2) x 2 x 108 m/s = 3 x 108 m/s

Speed of light in water = (3 x 108 m/s)/(4/3)
                                        = 2.25 x 108 m/s

Answer 2

4/3=c/2×10^8 (Cross multiply)
C=8×10^8/3=2.66×10^8m/s{c means speed of light in vacuum}
3/2=2.66×10^8/speed of light in water(nw)
nw=2.66×10^8×2/3=2.44×10^8 m/s