Discussion
If a thin lens of focal length 20 cm is in contact with another lens of power 4 D, then the effective power of the combination is
Answer: A
The effective power is 9 D.
Power in dioptres is the reciprocal of focal length in metres. For lenses in contact, the powers add:
\[ \begin{aligned} P_{1} &= \frac{1}{0.20\ \text{m}} = 5\ \text{D} \\ P &= P_{1} + P_{2} = 5 + 4 = 9\ \text{D} \end{aligned} \]
5 D is the first lens alone. The focal length must be in metres, not centimetres, or the power comes out a hundred times too small. 6, 10 and 11 D do not follow from these values.
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