Question

an object same size is obtained on the screen placed at 40 cm from it what is the nature of lens predict its focal length
plssss fasttttt!!​

Answer 1

Answer:

concave ,-20cm

Explanation:

m=1

1=v/u

v=40cm

using lens formula

f= -2/40 cm ,f=-20 cm

concave -20cm

Answer 2

Given:

An object of same size is obtained on the screen placed at 40 cm.

To find:

• Nature of lens

• Focal length of lens.

Calculation:

An image of the same size as that of the object can only be formed by a bi convex lens.

• In this type of cases , the image is inverted, real and has a magnification of -1.

$\rm{ \therefore \: magnifiction = - 1}$

$\rm{ = > \dfrac{v}{u} = - 1}$

$\rm{= > v = - u}$

Now , applying Len's Formula:

$\rm{ \therefore \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} }$

$\rm{ = > \dfrac{1}{f} = \dfrac{1}{( - u)} - \dfrac{1}{u} }$

$\rm{ = > \dfrac{1}{f} = - \dfrac{1}{u} - \dfrac{1}{u} }$

$\rm{ = > \dfrac{1}{f} = - \dfrac{2}{u} }$

$\rm{ = > f = - \dfrac{u}{2} }$

$\rm{ = > f = - \dfrac{( - 40)}{2} }$

$\rm{ = > f = 20 \: cm}$

So, focal length of lens is 20 cm.