Q1. A convex lens of focal length 30cm is combined with a concave lens of focal length 30cm. What is the power of the combined lens?(1)

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Solution

$f o c a l l e n g t h o f e a c h i s 30 c m . p o w e r o f c o n v e x l e n s p_{1} = \frac{100}{30} D p o w e r o f t h e c o n c a v e l e n s i s p_{2} = - \frac{100}{30} D p o w e r o f t h e c o m b i n a t i o n i s P = P_{1} + P_{2} P = \frac{100}{3} - \frac{100}{3} = 0 h e n c e p o w e r o f t h e c o m b i n t a i o n i s z e r o h e n c e f o c a l l e n g t h i s i n f i n i t y .$

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