Exams › JEE Main › Physics

Two lenses of power −15 D and +5 D are in contact with each other. The focal length of the combination is

1. +10 cm
2. −20 cm
3. −10 cm
4. +20 cm

Correct answer: −10 cm

Solution

The focal length of a lens combination can be calculated using the formula 1/f = 1/f1 + 1/f2. For the given powers, the focal lengths are -6.67 cm and +20 cm, respectively, leading to a combined focal length of -10 cm.

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 →