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The radius of curvature of a thin plano-convex lens is 10 cm (of curved surface) and the refractive index is 1.5. If the plane surface is silvered, then it behaves like a concave mirror of focal length
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- 10 cm
- 15 cm
- 20 cm
- 5 cm
Correct Option: A
The silvered plano convex lens behaves as a concave mirror ; whose focal length is given by
| = | + | |||||
| F | f1 | fm |
If plane surface is silvered
| fm = | = | = ∞ | ||
| 2 | 2 |
| ∴ | = (μ - 1) |  | - |  | ||||
| f1 | R1 | R2 |
| = (μ - 1) | | - | | = | ||||
| R | ∞ | R |
| ∴ | = | + | = | ||||
| F | R | ∞ | R |
| F = | ||
| 2(μ - 1) |
Here, R = 10 cm, µ = 1.5
| ∴ F = | = 10 cm | |
| 2(1.5 - 1) |
