First slide

The lens

Question

A thin convex lens of focal length 10 cm is placed in contact with a concave lens of same material and of same focal length. The focal length of combination will be

Moderate

A

Zero
B

Infinity
C

10 cm
D

20 cm

Solution

$f = \frac{f_{1} f_{2}}{f_{1} + f_{2}} = \frac{10 (- 10)}{10 + (- 10)} = \frac{- 100}{10 - 10} = \infty$

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