A thin plano-convex glass lens $\left(\mu =1.5\right)$ has its plane surface silvered and R is the radius of curvature of curved part, then which of the following ray diagram is true for an object placed at O? 

Focal length of the convex lens 
$\frac{1}{f}$     =  $\left(\frac{{\mu }_2-{\mu }_1}{{\mu }_1}\right)(\frac{1}{R_1}-\frac{1}{R_2})$
 $\frac{1}{f}$    =  $\left(\frac{1.5-1}{1}\right)(\frac{1}{R}-\frac{1}{\infty })$ = $\frac{1}{2R}$   $\Rightarrow$ f = 2R
 So the ray would become parallel to the principal axis after the refraction and fall ^r to the mirror and hence would get reflected back along the same path.