Question

# Calculate the focal length of the thin lens shown in the figure. The points C1 and C2 denote the centres of curvature. 20 cm40 cm60 cm80 cm

Solution

## The correct option is B 40 cmR1 = +10 cm and R1 = +20 cm The focal length is given by 1f=(μ−1)(1R1−1R2)=(1.5−1)(110 cm−120 cm)=0.5×120 cm=140 cmor, f=40 cm

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