| The convex surface of a thin concavo-convex lens of glass of refractive index 1.5 has a radius of curvature of \[20\,cm\]. The concave surface has a radius of curvature of \[60\,cm\]. The convex side is silvered and placed on a horizontal surface as shown in the figure. |
 |
| The focal length of the combination has the magnitude |
A) \[8.6\,cm\]
B) \[7.5\,cm\]
C) \[1.5\,cm\]
D) \[15\,cm\]
Correct Answer: B
Solution :
Use the equation \[\pm \frac{1}{f}=\frac{1}{{{f}_{1}}}\pm \frac{1}{{{f}_{2}}}+.....\] to find the equivalent power of the combination, with proper sign. Then use the equation of the lens/ mirror as appropriate.
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