Question

calculate the distance at which an object should be placed in front of a convex lens of focal length 10 cm to obtain an image double its size

Answer 1

Answer:

Given:

a convex lens

focal length (f) = 10cm

To find :

  the distance at which object should be placed so that image is of double size.

object distance =?

Formulas :

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

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

Solving Question:

  we are given the focal length and magnification = 2 ,so we have to find the relation b/w 'v' and 'u' the equate in the lens equation

Solution:

  $Magnification=\dfrac{v}{u}\\\\\implies 2 =\dfrac{v}{u} \implies v=2u$

v = 2u

$\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}\\\\\implies \dfrac{1}{2u}-\dfrac{1}{u}=\dfrac{1}{10}\\\\\implies \dfrac{1-2}{2u}=\dfrac{1}{10} \:\implies \dfrac{-1}{2u}=\dfrac{1}{10}\\\\\implies u =-5cm$

u = -5cm

Thus, the object distance should be -5cm

Answer 2

Answer:

-5cm

it's correct answer