First slide

Ray optics

Question

A linear object of height 5 mm is placed on the principal axis of a concave lens of focal length 20 cm at a distance of 20 cm from the lens and the object is perpendicular to the principal axis. Then size of its image is

Moderate

A

0.5 mm
B

7.5 mm
C

2.5 mm
D

4 mm

Solution

$\frac{1}{V} - \frac{1}{(- 20)} = \frac{1}{(- 20)} \Rightarrow V = - 10 c m$
$∴ \frac{size of image}{size of object} = \frac{V}{u} = \frac{- 10}{- 20}$
$\Rightarrow size of image = 5 \times \frac{1}{2} = 2.5 m m$

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