Ray Optics · Refraction
If sunlight is focused on a paper using convex lens (take the lens to be equiconvex), 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 . The value of is .
We corrected a printing error in the original paper
Assumption “(take the lens to be equiconvex)” added; the source leaves the lens shape unstated (a plano-convex lens would give ).
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