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Question
Solve the following example.
An object kept 60 cm from a lens gives a virtual image 20 cm in front of the lens. What is the focal length of the lens? Is it a converging lens or diverging lens?
Solution
Given:
Object distance, u = −60 cm
Image distance, v = −20 cm
Using lens formula,
\[\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\]
\[ \Rightarrow \frac{1}{f} = \frac{1}{- 20} - \frac{1}{- 60} = - \frac{1}{30}\]
\[ \Rightarrow f = - 30 \text{ cm }\]
Since, the focal length is negative, the lens is a diverging lens or a concave lens.
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