Refractive index (μ) = \(\frac{c(speed\,of\,light\,in\,vaccum)}{v(speed\,of\,light\,in\,any\,medium)}\)
Given μ = 2.42, c = 3 × 108.
Putting the value in the above formula, we get
⇒ 2.42 = \(\frac{3\,\times\,10^8}{v}\)
⇒ v = \(\frac{3\,\times\,10^8}{2.42}\)
⇒ v = 1.23 × 108 m/s.
Speed of light in diamond is 1.23 × 108 m/s
% of speed in diamond = \(\frac{1.23\,\times\,10^8}{3\,\times\,10^8}\)x 100 = 0.41 x 100 = 41%
Thus there is 41% decrease in the speed of light in diamond than in air.