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In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin)? n1=1.5n_1 = 1.5, n2=1.6n_2 = 1.6, R1=R2=20R_1 = R_2 = 20 cm

A

40 cm

B

-40 cm

C

-100 cm

D

-50 cm

Step-by-Step Solution

The system consists of two plano-convex lenses and one biconvex lens. The equivalent power P=P1+P2+P3P = P_1 + P_2 + P_3. Using the lens maker's formula 1f=(n−1)(1R1−1R2)\frac{1}{f} = (n-1)(\frac{1}{R_1} - \frac{1}{R_2}), the calculation leads to f=−100f = -100 cm.

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