The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is __________ cm.
Answer (integer)
15
Solution
${1 \over v} - {1 \over u} = {1 \over f}$ .... (1)
<br><br>m = ${v \over u}$ ..... (2)
<br><br>from (1) and (2) we get
<br><br>m = ${f \over {f + u}}$
<br><br>given conditions
<br><br>m<sub>1</sub> = -m<sub>2</sub>
<br><br>${f \over {f - 10}} = {{ - f} \over {f - 20}}$
<br><br>$\Rightarrow$ f – 20 = -f + 10
<br><br>$\Rightarrow$ 2f = 30
<br><br>$\Rightarrow$ f = 15 cm
About this question
Subject: Physics · Chapter: Optics · Topic: Reflection and Mirrors
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