Question

an object 2 m from the lens forms an erect image 1/4 th size of the object . determine the focal length of the lens. what type of lens it is ?

Answer 1

Answer:

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Answer 2

✒Question :-

An object 2m from the lens forms an erect image 1/4th size of the object . Determine the focal length of the lens. What type of lens it it ?

✒Given :-

An object 2m from the lens forms an erect image 1/4 th size of the object.

✒To find :-

•Focal length of the lens

•Type of lens

✒Solution :-

Distance of object (u) = - 2m

v = ?

$\sf\implies\dfrac{1}{4}=\dfrac{v}{u}$

$\sf\implies\dfrac{1}{4} = \dfrac{v}{-2}$

$\sf\implies v = -\dfrac{2}{4}$

$\sf\implies\dfrac{-1}{2} = -0.5m$

✒By using lense formula :-

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

$\sf\implies\dfrac{1}{-0.5}-\dfrac{1}{-2}=\dfrac{1}{f}$

$\sf\implies-\dfrac{1}{0.5}+\dfrac{1}{2}=\dfrac{1}{f}$

$\sf\implies\dfrac{1}{f}=\dfrac{-3}{2}$

✒Focal length of the lens (f) = $\sf-\dfrac{2}{3} m$

✒Negative sign indicates that the lense is concave lens .