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

MCQ

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

✓
$+18.75 \mathrm{~cm}$ and convex lens
B
$-18.75 \mathrm{~cm}$ and concave lens
C
$+20.25 \mathrm{~cm}$ and convex lens
D
$-20.25 \mathrm{~cm}$ and concave lens

✓

Answer

Correct option: A.

$+18.75 \mathrm{~cm}$ and convex lens

$ u=-25 \mathrm{~cm}, v=+75 \mathrm{~cm} $
$\Rightarrow \frac{1}{f}=\frac{1}{+75}-\frac{1}{-25} $
$\Rightarrow f=+18.75 \mathrm{~cm} ; \text { convex lens. }$

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