Question

The length of a real image of an object is twice the length of the object, placed at 25cm from a convex lens. Find the power of the convex lens.

Answer 1

Power of the lens is 50 D.

Explanation:

Let h is the size of object and h' is the size of image such that,

h' = 2h

Object distance, u = -25 cm

Let v is the image distance. The formula of the magnification of lens is given by :

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

$\dfrac{v}{-u}=2$

$\dfrac{v}{-25}=2$

v = -50 cm

Using lens formula to find f as :

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

$\dfrac{1}{-50}-\dfrac{1}{-25}=\dfrac{1}{f}$

$\dfrac{1}{f}=50$        

Power, $P=\dfrac{1}{f}$

$P=\dfrac{1}{f}=50\ D$    

So, the power of the convex lens is 50 D.

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Topic : Lens formula

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