Question

a teacher uses a convex lens of focal length 20 cm in a projector to project the image of a slide on the laboratory wall.she kept the projector at a distance of 70 cm from 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

this is a confirmed answer