Discussion
A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is
Answer: A
The power of the combination is -1.5 D.
Power in dioptres is the reciprocal of the focal length in metres, positive for a convex lens and negative for a concave lens. For lenses in contact, the powers add:
\[ \begin{aligned} P &= \frac{1}{0.40} + \left(-\frac{1}{0.25}\right) \\ &= 2.5 - 4 = -1.5\ \text{D} \end{aligned} \]
+1.5 D ignores the fact that the stronger concave lens wins, and ±6.5 D comes from adding the two powers as if both had the same sign.
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