First slide

The optical instruments

Question

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

Moderate

A

+ 1.5
B

– 1.5
C

+ 6.67
D

– 6.67

Solution

$Power of convex lens P_{1} = \frac{100}{40} = 2 .5 D Power of concave lens P_{2} = - \frac{100}{25} = - 4 D Now P = P_{1} + P_{2} = 2 .5 D - 4 D = - 1 .5 D$

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