One side of the bi-concave lens is silvered. It behaves like 
 

 

Concave mirror of focal length $=\frac{\mathrm{R}}{4\mathrm{\mu }-2}$

Convex mirror of $\mathrm{f}=\frac{\mathrm{R}}{2\mathrm{\mu }}$

Convex mirror of focal length $=\frac{\mathrm{R}}{4\mathrm{\mu }-2}$

Concave mirror of $\mathrm{f}=\frac{\mathrm{R}}{2\mathrm{\mu }}$