IIT JEE , AIEEE Forum - Physics , Mechanics

rakesh61

A sheet placed 90 cm from an object

the image of the obj on the screen is formed by a convex lens at 2 different location on the separated by 20 cm

Determine the focal length of the lens

Ans 21.4

Please give detailed solution rates assured

raulrag009

Let the convex mirror be placed x in front of the obj.

u = -x

v= (90-x)

f=f

1/v -1/u=1/f

solve u get

90f=90x-x²............(1)

Now for the new pos of mirror

u =-(x+20)

v=70-x

f=f

use the lens formulae again

u get

90f=(70-x)(x+20).................(2)

from eqn (1) and (2)

90x-x²=70x+1400-x²-20x

90x=50x+1400

40x=1400

x=35

Put this value in any eqn u get

90f=3150-1225

f=1925/90 = 21.38(approx)