Question

The image of an object placed at 40 cm in front of a lens is obtained on a screen is at a distance of 100 cm from it find the focal length of the lens what would be the height of the image of the object is 4 cm high

Answer 1

which lens it is pls tell clearly

Answer 2

Given that,

Object distance from a lens, u = -40 cm

The image is obtained on a screen is at a distance of 100 cm from it, v = 100 cm

To find,

The focal length of the lens and the height of the image.

Solution,

Using lens formula to find the focal length of the lens such that,

$\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}\\\\\dfrac{1}{100}-\dfrac{1}{(-40)}=\dfrac{1}{f}\\\\f=28.57\ cm$

The focal length of the lens is 28.57 cm.

Magnification,

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

h' is height of the image

$h'=\dfrac{vh}{u}\\\\h'=\dfrac{100\times 4}{-40}\\\\h'=-10\ cm$

So,the height of the image is (-10 cm).

Learn more,

Lens formula

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