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A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close focal contact with each other. Calculate the lens power of this combination.

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Given; 

Focal Length of convex lens f1 = 25cm = 0.25m (1m = 100cm); 

Focal Length of concave lens f2 = -10cm = -0.1m (1m = 100cm); 

Power of convex lens P1\(\frac{1}{f_1}=\frac{1}{0.25}=\frac{1}{\frac14}\)

∴ P1 = 4D. 

Power of concave lens P2\(\frac{1}{f_2}=\frac{1}{-0.1}=\frac{-1}{\frac1{10}}\)

∴ P2 = -10D. 

Power of combination = P1 + P2 = 4-10 = 6D. 

Thus, power of combination is -6 Dioptre.

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