Lense.....problem...

When one surface of a biconvex lens is silvered then in this situation , Two times refraction through lens and one time reflection occurs
So power of combination
P = P_L+P_m+P_L = 2P_L+P_m
P_m=-1f_m , f_m=R2 , P_L = 1f_L (f_L is in metre )

for plano convex lens
two cases may arise
CASE-I
If plane surface is silvered
let radius of curvature is R and μ be its refractive index
P = 2P_L + P_m
P_L = 1f_L = (μ-1)\left[\frac{1}{R}-\frac{1}{-\infty } \right]
\Rightarrow P_{L} = \frac{(\mu -1)}{R}

P_M = -1f_m = -1∞ = 0

hence P = 2(μ-1)R , f = R2(μ-1)

CASE - II
if curved surface is silvered
P = 2P_L+P_m

P_{L} = (\mu -1)\left[\frac{1}{\infty } -\frac{1}{-R}\right] = \frac{(\mu -1)}{R}

P_m = -1f_m = -1-R/2 = 2R

hence P = 2μR and f = R2μ

only first cassse was asked to bataya kyun nahin
Bohat type karna pada bekar main :P
but nevertheless if it helped u then its fine :)