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When a pin of height 3 cm is fixed at 10...

When a pin of height 3 cm is fixed at 10 cm from a conves lens, the height of the virtual image formed is 12 cm. Find the focal length of the lens.

Text Solution

Verified by Experts

`h_(2)/h_(1) = v/u`
` therefore (12cm)/(3 cm) = v/( -10 cm) " " therefore v = -40 cm`
Now ` 1/f = 1/v - 1/u`
` therefore 1/f = 1/( - 40 cm) = 1/( - 10 cm) = 1/(10 cm) - 1/( 40 cm) `
= `( 4-1)/( 40 cm) = 3/( 40 cm)`
` f = 40/ 3 ""cm = 13.33` cm ( approximately) '
The focal length of the lens = 13.33 cm [ approximately].
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