First slide

The lens

Question

Two thin lenses of focal lengths f₁ and f₂ are in contact. The focal length of this combination is

Moderate

A

$\frac{f_{1} f_{2}}{f_{1} - f_{2}}$
B

$\frac{f_{1} f_{2}}{f_{1} + f_{2}}$
C

$\frac{2 f_{1} f_{2}}{f_{1} - f_{2}}$
D

$\frac{2 f_{1} f_{2}}{f_{1} + f_{2}}$

Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} F = \frac{f_{1} f_{2}}{f_{1} + f_{2}}$

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