Question

Calculate the distance at which an object should be placed in front of a convex lens of focal length 100cm to obtain an erect image of double it's size

Answer 1

let size of image be 2h cm and size of object be h cm

we know that,

h2/h1=v/u

2h/h=v/u

v=2u. eq:-(i)

given, f=100 cm

by lens formula

1/f=1/v-1/u

1/100=1/2u-1/u. from -i

1/100=-1/2u

-2u= -100

therefore u=50 cm

hence object distance is 50cm