# A concave lens of focal length 25 cm and a convex lens of focal length 20 cm are placed in contact with eachother. What is the power of this combinati

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A concave lens of focal length 25 cm and a convex lens of focal length 20 cm are placed in contact with eachother. What is the power of this combination ? Also, calculate focal length of the combination.

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Here, focal length of concave lens, f_(1) = -25 cm, focal length of convex lens, f_(2) = + 20 cm
power of the combination, P = ?, focal length of the combination, F = ?
Clearly, P_(1) = 100/(f_(1)(cm)), P = 100/(-25) = -4 dioptre and P_(2) = 100/(f_(2)(cm)) = 100/20 = + 5 dioptre
As P = P_(1) + P_(2), P = - 4 + 5 = 1 dioptre
Thus, F = 100/P = 100 cm = 1 m.
Positive sign of P and F indicates that the combination of two given lenses behaves as a convex lens.