A spherical convex surface separates object and image space of refractive index 1 and 4/3 respectively. If radius of curvature of the surface is 0.1 m, its power is : 

2.5 D 

$ u \to \infty , v = f$

$ \frac{\mu_2}{v} - \frac{\mu_1}{u} = \frac{\mu_2 - \mu_1}{R}$

$ \frac{\mu_2}{f} = \frac{\mu_2 - \mu_1}{R}$

$\frac{4}{3f} = \frac{1}{3R}$ 

$ \Rightarrow P = \frac{1}{f} = \frac{1}{4R} = \frac{1}{0.4} = 2.5D$