Question

A convex lens has focal length of 20 cm. At what distance from the lens should the object be placed so that the image is formed at 40 cm on the other side of the lens? a. 40 Cm b. 20cm c. -40 CM d. -20 cm​

Answer 1

$here$$focal$$length$$is$$positive$$f= 20 cm$

$here$$v$$is$$also$$positive$$v = 40 cm$

$u= ?$

$\frac{1}{v } - \frac{1}{u} = \frac{1}{f}$

$\frac{1}{40} - \frac{1}{u} = \frac{1}{20}$

$\frac{1} {40} - \frac{1}{20} = \frac{1}{u}$

$\frac{1 - 2}{40} = \frac{1}{u}$

$\frac{ - 1}{40} = \frac{1}{u}$

$u = -40 cm$

hope it help you