First slide

The lens

Question

If two + 5 D lenses are mounted at some distance apart, the equivalent power will always be negative if the distance is

Moderate

A

Greater than 40 cm
B

Equal to 40 cm
C

Equal to 10 cm
D

Less than 10 cm

Solution

$P = P_{1} + P_{2} - {dP}_{1} P_{2} \Rightarrow P = 10 - 25 d \Rightarrow For P to be negative 25 d > 10 \Rightarrow d > 0 .4 m or d > 40 cm$

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