Question

A real image 2/3 rd of the size of an object is formed by convex lens when the object is at a distance of 12 cm from it. Find focal length

Answer 1

M = 2/3

u = -12cm

Since we know that M=v/u

Therefore,

v= 8cm

Since by mirror formula,

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

There fore

F = +4.8cm

Answer 2

Answer:

v = -18 cm and f = 36 cm

Explanation:

Common Misconception - the object is at a distance of 12 cm from it

i.e u = -12 cm and not v = -12 cm

A real image 2/3 rd of the size of an object

i.e (2/3)h' = h

Therefore m = h'/h = 3/2 ----------{1}

sub {1} in h'/h = v/u

you get v = -18 cm

sub v = -18 cm in lens formula 1/f = 1/v- 1/u

we get f = 36 cm