Physics - Geometrical Optics Question with Solution | TestHub

A concave lens is required by the person so that the lens forms the image of object distant 30 cm , at 10 cm it means $\begin{array}{rl}u& =-30\mathrm{cm},v=-10\mathrm{cm}\\ \frac{1}{f}& =\frac{1}{v}-\frac{1}{u}\\ & =\frac{1}{-10}-\frac{1}{-30}=-\frac{2}{30}\\ \text{ or }\frac{1}{f}& =\frac{-1}{15}\\ \text{ So }f& =-15\mathrm{cm}\end{array}$ Minus sign singnifies that lens used is concave