Question

For a convex lens, focal length is 20 cm. If u = - 20 cm, the image distance will be?

Answer 1

$focal \: length(f) = 20( conex \: lens) \\ u = - 20 \\ \\ lens \: formula \: = \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \\ \frac{1}{20} = \frac{1}{v} -\frac{1}{ (- 20)} \\ \frac{1}{20} + \frac{1}{20} = \frac{1}{v} \\ \frac{2}{20} = \frac{1}{v} \\ v = \frac{20}{2} \\ v = 10 \\ image \: will \: form \: 10cm from \: lens$

Hope it is helpful