An object is placed at a distance of 30 cm in front of a convex lens. What is the focal length of the lens if the image is formed at a distance of 60 cm on the other side of the lens?

+20 cm

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-20 cm

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-15 cm

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+15 cm

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Solution

The correct option is A +20 cm
Given object distance, u = -30 cm.
Image distance = +60 cm.

Using lens formula,
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\Rightarrow \frac{1}{f} = \frac{1}{+ 60 c m} - \frac{1}{(- 30 c m)}$
$\Rightarrow f = + 20 c m$

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An object is placed at a distance of 30 cm in front of a convex lens. What is the focal length of the lens if the image is formed at a distance of 60 cm on the other side of the lens?

The correct option is A +20 cmGiven object distance, u = -30 cm. Image distance = +60 cm. Using lens formula, 1f=1v−1u ⇒1f=1+60 cm−1(−30 cm) ⇒f=+20 cm

The correct option is A +20 cm
Given object distance, u = -30 cm.
Image distance = +60 cm.

Using lens formula,
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\Rightarrow \frac{1}{f} = \frac{1}{+ 60 c m} - \frac{1}{(- 30 c m)}$
$\Rightarrow f = + 20 c m$