Question

an object is put at a distance 25cm from the optical centre of a convex lens. if its real image is formed at a distance 30cm from the lens then its focal length is about​

Answer 1

Answer:

Focal length, f = 13.63 cm

Explanation:

It is given that,

Object distance, u = -25 cm

Its real image is formed at a distance of 30 cm i.e. image distance, v = +30 cm

We have to find the focal length of the lens. It can be calculated suing lens formula. It can be written as :

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

$\dfrac{1}{30\ cm}-\dfrac{1}{-25\ cm}=\dfrac{1}{f}$

f = 13.63 cm

So, the focal length of the convex lens is 13.63 cm.

Answer 2

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➠ Given

• Object distance from the lens (u) = -25 cm

• Image is real and inverted

• Image Distance from the lens (v) = + 30 cm

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➠ To Find

We have to find the focal length of the convex lens.

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➠ Solution

As, we have to find the focal length of the convex lens.

So, we will use lens formula

$\Large{\star{\boxed{\sf{\frac{1}{v} - \frac{1}{u} = \frac{1}{f}}}}}$

(Putting Values)

$\sf{\implies \frac{1}{30} - \frac{1}{(-25)} = \frac{1}{f}} \\ \\ \sf{\implies \frac{1}{30} + \frac{1}{25} = \frac{1}{f}} \\ \\ \sf{\implies \frac{5 + 6}{150} = \frac{1}{f}} \\ \\ \sf{\implies \frac{11}{150} = \frac{1}{f}} \\ \\ \sf{\implies f = \frac{150}{11}} \\ \\ \sf{\implies f = 13.63}$

∴ Focal length of the convex lens is 13.63 cm.