A convex lens of focal length 10 cm is placed in closed contact with a concave lens of focal length 10 cm. The power of combination is 20 D
10 D
4D
Zero
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Answers
f1 = 25cm (Focal length of convex lens is positive) ⇒ f1 = 0.25m (1m = 100cm) P (in dioptre) = 1 f ( i n m e t e r s ) 1f(inmeters) ⇒ Power of convex lens,(F=25 cm) P1 = 1 f 1 1f1 ⇒ P1 = 1 0.25 = 1 1 4 10.25=114 ∴ P1 = 4 D f2 = -10cm (Focal length of concave lens is negative) ⇒ f2 = -0.1m (1m = 100cm) ⇒ Power of the concave lens (F=10 cm), P2 = 1 f 2 1f2 ⇒ P2 = − 1 0.1 = 1 1 10 −10.1=1110 ∴ P2 = -10 D. Power of combination = P1 + P2. P = 4D – 10D = -6D. Lens power of the combination is -6D. Since it is negative therefore the new lens will behave like concave lens having a focal length of 16.6cm.
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