Question

at what position a candle of length 3cm be placed in front of a convex lens so that its image of length 6cm be obtained on a screen placed at distance 30cm behind the lens

Answer 1

height of the object h1=3cm
height of the image h2 =-6cm( the image is formed below the principal axis)
v= -30cm
m=h2/h1  =-6/3= -2
m=v/u   -2=30/u
u=-15 cm
on calculation the focal length can be calculated by the formula 1/f=1/v-1/u
so f = 10cm
so the object is placed between F1 and 2F1 and the image is formed beyond 2F2.

Answer 2

Answer:height of the object h1=3cm

height of the image h2 =-6cm( the image is formed below the principal axis)

v= -30cm

m=h2/h1  =-6/3= -2

m=v/u   -2=30/u

u=-15 cm

on calculation the focal length can be calculated by the formula 1/f=1/v-1/u

so f = 10cm

so the object is placed between F1 and 2F1 and the image is formed beyond 2F2.