JEE Mains · Physics · STD 12 - 9. Ray optics and optical instruments
If sunlight is focused on a paper using convex lens, it starts burning the paper in shortest time when the lens is kept at \(30\) cm above the paper. If the radius of curvature of the lens is \(60\) cm then the refractive index of the lens material is \(\dfrac{\alpha}{10}\). The value of \(\alpha\) is _______.
- A 20
- B 24
- C 26
- D 28
Answer & Solution
Correct Answer
(A) 20
Step-by-step Solution
Detailed explanation
Sunlight comes from infinity and is focused at the principal focus of the convex lens. The paper burns in the shortest time when the intensity of light is maximum, which occurs at the focal point. Therefore, the focal length of the convex lens is \(f = 30\) cm. Assuming the lens…
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