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  4. The figure shows a thin lens with point C1 and C2 as the centres of curvature. The focal length of lens will be (refractive index = 1.5)

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Q. The figure shows a thin lens with point $ {{C}_{1}} $ and $ {{C}_{2}} $ as the centres of curvature.
The focal length of lens will be (refractive index = 1.5)Physics Question Image

VMMC MedicalVMMC Medical 2015

Solution:

The radii of the curvature are positive Thus, $ {{R}_{1}}=+15cm $ and $ {{R}_{2}}=+20cm $ The focal length is given by $ \frac{1}{f}=(\mu -1)\left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right] $ $ \Rightarrow $ $ \frac{1}{f}=(1.5-1)\left[ \frac{1}{15}-\frac{1}{20} \right] $ $ =0.5\times \frac{1}{60}=\frac{5}{600} $ $ \Rightarrow $ $ f=\frac{600}{5}=120cm $