Refractive index of water with respect to air is 1.33 and that of diamond is 2.42. 

Refractive index = \(\frac{speed\,of\,light\,in\,vacuum}{speed\,of\,light\,in\,medium}\)

Since the refractive index of diamond is more, hence the speed of light is lesser in diamond. 

Let speed of light in water vev_w and in diamond bev_d. 

Refractive index of diamond w.r.t water is say n = \(\frac{speed\,of\,light\,in\,water}{speed\,of\,light\,in\,diamond}\)

\(n=\frac{V_w}{V_d}\)

Dividing both numerator and denominator by speed of light [c] we get 

n = (\(\frac{V_w}{c}\)) ÷ (\(\frac{V_d}{c}\)) = Inverse Ratio of refractive index of water and diamond. 

n=\(\frac{2.42}{1.33}\) = 1.82 (approx)