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It is useful to design lenses of derived focal length using surface of suitable radii of curvature. Lens maker formula helps in finding various unknowns and is widely applied.

The radii of curvature of faces of a double concave lens are 20 cm and 30 cm. If refractive index of glass is $\frac{3}{2}$, than the focal length of the lens will be:

-24 cm

The correct answer is option (2) : -24 cm

From lens maker's formula

$\frac{1}{f}=(\mu - 1) \left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

$=\left(\frac{3}{2}-1\right)\left(-\frac{1}{20}-\frac{1}{30}\right)$

$=\frac{1}{2}\left(\frac{-3-2}{60}\right)=-\frac{1}{24}$

$f= - 24 cm $