A focal length of a thin biconvex lens is 20 cm. When an object is moved from a distance of 25cm in front of it to 50cm, the magnification of its image changes from $m_{25}\mathrm{t}\mathrm{o}\mathrm{ }{m}_{50}.$The ration $\frac{m_{25}}{m_{50}}$ is

Define power of a lens and write down its unit. For two coaxially contacted lenses, derive the relation : $\frac{1}{\mathrm{f}}=\frac{{\displaystyle 1}}{{\displaystyle {\mathrm{f}}_1}}=\frac{{\displaystyle 1}}{{\displaystyle {\mathrm{f}}_2}}$.

Find relation between $u, v$ and $f$ for a lens assuming it to be surrounding by two spherical surfaces.