Question

an object is 60 CM tall is placed on the axis of convex lens it is 30 cm image is formed on the screen placed at 40 cm from the object . then find the focal length.

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Answer 1

u + v = 40 cm

h = 60 cm

h' = 30 cm

m = h'/h=v/u

=> -60/30=v/u

v= -2u

u+v=40

-u-2u =40

u=-40/3

v=120-40/3

v=80/3

Answer 2

Explanation:

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An object is 60 CM tall is placed on the axis of convex lens it is 30 cm image is formed on the screen placed at 40 cm from the object. Then find the focal length.

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‌Given that,

Height of the object h = 60 cm

Height of image h' = 30 cm

Distance of the object v= 40 cm

The magnification is

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

$m = - \frac{v}{u} = \frac{30}{60}$

$\frac{ - 40}{u} = \frac{1}{2}$

$u = - 80cm$

Using lens's formula

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

$\frac{1}{f} = \frac{1}{40} + \frac{1}{80}$

$\frac{1}{f} = \frac{3}{80}$

$f = \frac{80}{3}$

$f = 26.67cm$