First slide

The lens

Question

If two lenses of +5 diopters 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

$Using P = P_{1} + P_{2} - d \times P_{1} P_{2} for equivalent power to be negative d \times P_{1} P_{2} > P_{1} + P_{2} \Rightarrow d \times 25 > 10 \Rightarrow d > \frac{10}{25} m \Rightarrow d > \frac{10 \times 100}{25} \Rightarrow d > 40 cm$

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