A candle placed 25 cm from a lens, forms an image on a screen pla… - Vidyadip

MCQ

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
B
$-18.75 cm$ and concave lens
C
$+ 20.25 cm$ and convex lens
D
$-20.25 cm$ and concave lens

✓

Answer

Correct option: A.

$+ 18.75 cm$ and convex lens

a
(a) $u = - \,25\,\,cm,\,v = + \,75\,cm$
$ \Rightarrow $$\frac{1}{f} = \frac{1}{{ + 75}} - \frac{1}{{ - 25}} \Rightarrow f = + 18.75 cm $ ; convex lens.

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