JEE Main 2026 · 2 Apr · Shift 2 · Physics Q49

← JEE Main 2026 · 2 Apr · Shift 2

PhysicsInteger+4 / -1Question 49 of 75

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 ________.

Your answer

Answer and worked solution

Official answer: 20

Focal length. The paper burns fastest at the focus, so $f=30$ cm.

Lens-maker (equiconvex, $R=60$ cm):
$\frac1{30}=(\mu-1)\frac2{60}\implies\mu=2=\frac{20}{10}.$

Answer: 20

Topics:Thin lenses and lens maker's formula

← Previous question Next question →

JEE Main 2026 · 2 Apr · Shift 2, Physics question 49