Figure shows three lenses of equal radii of curvature of the curved surfaces. The ratio of focal lengths of P, Q and R is

−1 : 2 : −1

For $\mathrm{P} \frac{1}{\mathrm{f}}=[\mu-1]\left[\frac{1}{\mathrm{R}}\right] \mathrm{f}_1=\frac{-\mathrm{R}}{2(\mu-1)}$ (since one side is silvered)

$\left.f_{e q}=\left(2 \frac{1}{f_L}+\frac{1}{f_m}\right) f_m \rightarrow \infty\right)$

for $Q \frac{1}{f}=[\mu-1]\left[\frac{1}{\infty}-\frac{1}{-R}\right] \mathrm{f}_2=\left(\frac{\mathrm{R}}{\mu-1}\right)$

For $R \frac{1}{f}=[\mu-1]\left[\frac{1}{-R}-\frac{1}{R}\right] f_3=\frac{-R}{2[\mu-1]}$