A candle placed 25 cm from a lens, forms an image on a screen placed 75 cm on the other end of the lens. The focal length and type of the lens should be

+ 18.75 cm and convex lens

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- 18.75 cm and concave lens

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+ 20.25 cm and convex lens

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- 20.25 cm and concave lens

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Solution

The correct option is A + 18.75 cm and convex lens
In concave lens, image is always formed on the same side of the object. Hence the given lens is a convex lens for which u = – 25 cm, v = 75 cm.
By using $\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{(+ 75)} - \frac{1}{(- 25)} \Rightarrow f = 18.75 c m .$

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A candle placed 25 cm from a lens, forms an image on a screen placed 75 cm on the other end of the lens. The focal length and type of the lens should be

The correct option is A + 18.75 cm and convex lensIn concave lens, image is always formed on the same side of the object. Hence the given lens is a convex lens for which u = – 25 cm, v = 75 cm. By using 1f=1v−1u⇒1f=1(+75)−1(−25)⇒f=18.75cm.