Solution:

Given,

According to the question,

speed of light in diamond is = 125,000 km s^-1 

Or, = 125 × 10⁶ m / s

We know that the speed of light in air is

c = 3 × 10⁸ m / s

We have to find the refractive index of diamond, μ_D

Expression which relates the refractive index wit the speed in mediums is given below –

μ = c / v

Upon putting values, => μ = (3 × 10⁸) m / s / (125 × 10⁶) m / s

Therefore, μ = 2.4