Exams › JEE Main › Physics

A thin convex lens made of crown glass with refractive index 3/2 has focal length f in air. If the same lens is immersed separately in two liquids of refractive indices 4/3 and 5/3, its focal lengths become f1 and f2, respectively. Which of the following correctly describes the relation among these focal lengths?

1. f1 = f2 < f
2. f1 is greater than f and f2 becomes negative
3. f2 is greater than f and f1 becomes negative
4. Both f1 and f2 become negative

Correct answer: f1 is greater than f and f2 becomes negative

Solution

When the lens is immersed in a liquid with a refractive index lower than that of the lens (like 4/3), its focal length increases, making f1 greater than f. However, when immersed in a liquid with a higher refractive index (5/3), the lens behaves as if it is diverging, resulting in a negative focal length for f2.

Related JEE Main Physics questions

• A telescope is fitted with an objective lens of focal length 100 cm and an eyepiece of focal length 5 cm. If the final image is adjusted to form at the least distance of distinct vision, what is the telescope’s magnifying power?
• An object placed 2.4 m in front of a lens produces a clear image on a film located 12 cm behind the lens. A 1 cm thick glass slab of refractive index 1.50 is then inserted between the lens and the film, with its plane surfaces parallel to the film. To obtain a sharp image again on the film, to what distance from the lens must the object be moved?
• A light ray is incident on an equilateral prism in such a way that the angle of incidence equals the angle of emergence, and this common angle is three-fourths of the prism angle. What is the angle of deviation produced?
• A glass lens with refractive index 1.60 has a focal length of +20 cm when placed in air. If the same lens is immersed in water of refractive index 1.33, its focal length will be
• A prism is placed in air. If a ray is incident at 60° and the prism angle, the deviation angle δ, and the angle of emergence e are all equal, what is the refractive index of the prism?
• A person’s farthest point of clear vision is 30 cm from his eyes. If he wants to read a book held 50 cm away, what power of lens should he use?

⚔️ Practice JEE Main Physics free + battle 1v1 →