First slide

The lens

Question

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

Moderate

A

+ 18.75 cm and convex lens
B

– 18.75 cm and concave lens
C

+ 20.25 cm and convex lens
D

– 20.25 cm and concave lens

Solution

$u = - 25 cm, v = + 75 cm \Rightarrow \frac{1}{f} = \frac{1}{+ 75} - \frac{1}{- 25} \Rightarrow f = + 18 .75 cm . convex lens.$

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