µ = 2.41; Ca = 3 × 108 ms [Velocity of light]
µ = \(\frac{Speed\, of\, light\, in\, air}{Speed\, of\, light\,in\, diamond}\) = \(\frac{C_a}{C_d}\)
Cd = \(\frac{c_a}{µ}\) = \(\frac{3 \times 10^8}{2.41}\) = 1.245 x 108 m/s
∴ Speed of light decreases when the light ray travels from air to diamond.