Question

A teacher uses of convex lens of focal length 20 CM in a project to project the image of a slide on the laboratory wall. She kept the projector at a distance of 70 CM from the wall .Find the distance between the lens and the slide

Answer 1

given, f=+20cm,here,u is unknown. and v=-70cm .so by lens formula,1/v-1/u=1/f
or, 1/-70-1/u=1/20
or,1/u=-1/20-1/70
or,1/u=1/-15.5cm.so,u=-15.5cm

Answer 2

Let's see given data.

Focal length(f) = 20 cm
Distance(u)      = -70 cm
v                       = ?

We know Lens formula:

$\frac{1}{f} = \frac{1}{u}- \frac{1}{v}$
$\frac{1}{20} = \frac{1}{-70}- \frac{1}{v}$
$\frac{1}{20} = -\frac{1}{70}- \frac{1}{v}$
$\frac{1}{20} + \frac{1}{70} = - \frac{1}{v}$
$\frac{7+2}{140} = - \frac{1}{v}$
$\frac{9}{140} = - \frac{1}{v}$
$v = \frac{140}{9}$
$v = 15.55$