Question

An object of height 4 cm is placed at a distance of 20 cm from the convex lens of focal length 10 centimetre use length formula to determine the position of this form

Answer 1

Given Information :

object distance from the lens (u) = 20 cm

image distance from the lens (v) = ?

focal length (f) = 10 cm

For a convex lens :

u is negative .

f is positive .

v is positive too ( when object is far away ) .

By Len's formula : 1/f = 1/v - 1/u

                               ==> 1/f = 1/v + 1/u [ since u is negative ]

                               ==> 1/v = 1/f - 1/u

                               ==> 1/v = 1/10 - 1/20

                               ==> 1/v = ( 2 - 1 ) / 20

                               ==> 1/v = 1/20

                               ==> v = 20

Hence v = 20 cm

Since it is positive image is behind the lens.

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$\boxed{\bf{ANSWER}}$

The image is formed 20 cm behind the lens !

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Hope it helps you :-)

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