Question

A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal lengh 20 cm . the distance of the object from the lens is 15 cm . Find the nature, position and the size of the image .

Answer 1

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Answer 2

The image is formed at a distance of 60 cm. The formed image is virtual and erect, formed between c and f, image is enlarged.

Explanation:

Height of the object is, h = 4 cm

Focal length of the convex lens, f = +20 cm ( positive for convex lens)

Object distance from the lens, u = -15 cm (negative always)

If v is the position of the image formed. It is required to find it using lens formula :

$\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}\\\\\dfrac{1}{v}=\dfrac{1}{f}+\dfrac{1}{u}\\\\\dfrac{1}{v}=\dfrac{1}{(20)}+\dfrac{1}{(-15)}\\\\v=-60\ cm$

Magnification of the lens is :

$m=\dfrac{v}{u}\\\\m=\dfrac{-60}{-15}\\\\m=4$

So, the formed image is virtual and erect, formed between c and f, image is enlarged.

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Lens formula

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