Physics Question 49 – JEE-MAIN 2026

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 $\frac{\alpha }{10}$. The value of $\alpha$ is _______.