Question

Find the focal length of a convex lens is 10 centimetre. A candle of height 12 centimetre is placed at a distance of 20 cm from that lens. The position, nature and length of image of candle?

Answer 1

Answer:

H^i = height of image

H^o=height of object

Answer 2

The height of the image is 12 cm and it is inverted.

Explanation:

It is given that,

Focal length of the convex lens, f = +10 cm

Height of the object, h = 12 cm

Object distance, u = -20 cm

Let v is the distance of image from the center. It can be calculated using Lens formula as :

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

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

$\dfrac{1}{v}=\dfrac{1}{10}+\dfrac{1}{(-20)}$        

v = 20 cm

The image is formed at a distance of 20 cm from the lens. Let m is the magnification. So,

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

$\dfrac{20}{-20}=\dfrac{h'}{12}$

h' = -12 cm

Height of the image is 12 cm and it is inverted. Hence, this is the required solution.

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Lens formula

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