Question

an object is placed at a distance of 20cm from a convex lens the image is formed on the screen at a distance of 60cm from the lens find its focal length ​

Answer 1

Given data : An object is placed at a distance of 20cm from a convex lens the image is formed on the screen at a distance of 60cm the lens.

To find : Focal length of the convex lens ?

Solution : Here we use lens formula to find focal length of the convex lens.

${\bf {\rightarrow{\frac{1}{v} - {\frac{1}{u}} = {\frac{1}{f}}}}}$

where,

→ v = image distance from lens

→ u = object distance from lens

→ f = focal length of lens

According to given

→ v = 60 cm

→ u = - 20 cm

${\bf {\rightarrow{\frac{1}{60} - {\frac{1}{ - 20}} = {\frac{1}{f}}}}}$

${\bf {\rightarrow{\frac{1}{60} + {\frac{ 3}{ 60}} = {\frac{1}{f}}}}}$

${\bf {\rightarrow{\frac{4}{60} = {\frac{1}{f}}}}}$

${\bf {\rightarrow{\frac{1}{15} = {\frac{1}{f}}}} \: \: \: \: \: \: \: \: i.e.}$

${\bf {\rightarrow{{f = 15 \: cm}}}}$

Hence, focal length of lens is 15 cm.

More info :

Nature of image :-

Here, u < f hence, image virtual, upright and magnified .

Answer 2

Solution:-

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