A thin plano-convex lens acts like a concave mirror of focal length 0.2 m when silvered at its plane surface. Refractive index of the material of lens is 1.5. Radius of curvature of convex surface of the lens is :
0.1 m
0.2 m
0.4 m
0.8 m
A
0.4 m
B
0.1 m
C
0.2 m
D
0.8 m
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Solution
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Given,
Focal length, F=0.2m=20cm
Refractive index, μ=1.5
We know
1f=2fl+1fm
But, fm=∞
1f=2fl
fl=2×20=40cm
From lens maker's formula
1f=(μ−1)[1R1−1R2]
140=(1.5−1)[1R1−1∞]
⇒140=0.5R1
R1=0.5×40=20cm
R1=0.2m
Radius of curvature of convex surface =0.2m
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