First slide

Ray optics

Question

An eye specialist prescribes spectacles having a combination of a convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm. The power of this lens combination is

Moderate

A

+ 1.5D
B

– 1.5D
C

+ 6.67D
D

– 6.67D

Solution

$P_{eff} = P_{1} - P_{2}, P = \frac{100}{f} D; P_{eff} = \frac{100}{40} - \frac{100}{25} = - 1.5 D$

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