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Ques. 1JEE Main 2025, 22 January (Morning Shift)
Given is a thin convex lens of glass (refractive index ) and each side having radius of curvature R. One side is polished for complete reflection. At what distance from the lens, an object be placed on the optic axis so that the image gets formed on the object itself?
- R/
- R/(2 -1)
- R/(2 -3)
Answer
(2) R/(2 -1)
Solution
The light retraces its path only if it hits the silvered face normally, i.e. after refraction it moves along a radius, as if from the centre of curvature C (distance R).
[figure]
Lens: 1f=( -1) 2R and the lens image is at v=-R
1v- 1u= 1f - 1R- 1u= 2( -1)R
|u|= R2 -1
Answer: (2)