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Question
A thin converging lens of focal length 12 cm is kept in contact with a thin diverging lens of focal length 18 cm. Calculate the effective/equivalent focal length of the combination.
Solution
Focal length of converging lens is positive, hence
f1 = + 12 cm
Focal length of diverging lens is negative, hence
f2 = - 18 cm
Equivalent focal length of the combination is given by,
`1/"f" = 1/"f"_1 + 1/(- "f"_2)`
`1/"f" = 1/12 - 1/18`
`1/"f" = 1/36`
⇒ f = 36 cm
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