Question

An object 50 cm tall is placed on the principal axis of a convex lens it's 20 cm tall image is formed on the screen placed at a distance of 10 cm from the lens calculate focal length of lens

Answer 1

Ho = 50cm
Hi = -20. [sign convention]
V = 10cm
Using,
Hi/Ho = V/U.

PUT VALUES AND FIND U ( OBJECT DISTANCE).

AFTER FINDING DISTANCE OF OBJECT.

USING LENS FORMULA,

-1/U+1/V=1/F.


FIND F ( FOCAL LENGTH)

Answer 2

ho = 50 hi = 20 v = -10
hi/ho = v/u
20/50 =-10/u
u= -(10*50)/20
= -25
1/f=1/v –1/u
1/f= 1/10 -(-1/25)
1/f=1/10+1/25
1/f=(5+2)/50
1/f= 7/50
f=50/7
f=7.12cm