First slide

Ray optics

Question

Two convex lenses of focal lengths 20 cm and 30 cm are kept at a separation of 15 cm. Then power of the combination is

Moderate

A

6.12 D
B

2.13 D
C

5.83 D
D

4.5 D

Solution

Equivalent focal length F is give by
$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} f_{2}} = \frac{1}{20} + \frac{1}{30} - \frac{15}{20 \times 30} a m^{- 1}$
$\Rightarrow Power P = \frac{100}{F} D = \frac{100}{20} + \frac{100}{30} - \frac{15 \times 100}{20 \times 30} D = 5.83 D$

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