Two lenses o focal length +10cm and -5cm are kept in contact.What is the focal length and power of combination?
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Focal length of first lens, f1 = +20 cm
Focal length of second lens, f2 = -10 cm
Since the power and the focal length of any lens is related by the relation, P = 1/f
Power of first lens, P1 = 1/20 = 0.05 D
Power of second lens, P2 = 1/-10 = -0.1 D
When two lens of power P1 and P2 are made in contact with each other, the resultant power of the combination is P1 + P2
Hence, total power, P = 0.05D + (-0.1D) = -0.05 D
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