Question

An object 2 m tall is placed on the principal axis of a convex lens and its 40 cm tall image is formed on the
screen placed at a distance of 72 cm from the object. What is the focal length of the lens?​

Answer 1

The

answer

is

given

above

with

explanation

clearly

Answer 2

Answer:

Required focal length is 480 cm.

Explanation:

Given,height of the object is 2 cm height of the image is 40 cm and object distance is 72 cm.

Let height of object be h and height of image is h'.

So,h=2 cm and h'=40 cm.

We know Magnification,$m = \frac{h}{h'} = \frac{2}{40} = \frac{1}{20}$

Again we know,

$m = \frac{v}{u}$

Where v is object distance and u is image distance.

$m = \frac{72}{u} = \frac{1}{20}$

Therefore,$u = 72 \times 20 = 1440$cm

We know for convex mirror,

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

$f = \frac{uv}{u + v} = \frac{72 \times 1440}{72 + 1440} = 480$cm

Required focal length 480 cm.