# Numerical on Power of a Lens for CBSE Class 10

Q.1. A concave lens produces an image 20 cm from the lens of an object placed 30 cm from the lens. Calculate the power of the lens.

Q.2. A convex lens is of focal length 10 cm. What is its power?

Q.3. A person having a myopia eye uses a concave lens of focal length 50 cm. What is the power of the lens?

Q.4. A thin lens has a focal length of –25 cm. What is the power of the lens and what is its nature?

Q.5. A lens has a power of –2.5 D. What is the focal length and nature of the lens? 6.Find the power of a concave lens of focal length 2 m.

Q.6. A convex lens forms a real and inverted image of needle at a distance of 50 cm from the lens. If the image is of the same size as the needle, where is the needle placed in front of the lens? Also, find the power of the lens.

Q.7. Two thin lenses of power +3.5 D and –2.5 D are placed in contact. Find the power and focal length of the lens combination.

Q.8. A doctor has prescribed a corrective lens of power –1.5 D. Find the focal length of the lens. Is the prescribed lens is diverging or converging?

Q.9. A concave lens of focal length 25 cm and a convex lens of focal length 20 cm are placed in contact with each other. What is the power of this combination? Also, calculate focal length of the combination.

Q.10. A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 10 cm. What is the focal length and power of the combination?

Q.11. An object is placed at a distance of 50 cm from a concave lens of focal length 30 cm. Find the nature and position of the image.

Q.12. An object of height 2 cm is placed at a distance of 15 cm in front of a concave lens of power –10 D. Find the size of the image.

Q.13. A convergent lens of power 8 D is combined with a divergent lens of power –10 D. Calculate focal length of the combination.

Q.14. A concave lens is kept in contact with a convex lens of focal length 20 cm. The combination works as a converging lens of focal length 100 cm. Calculate power of concave lens.

Q.15. Find the focal length and nature of lens which should be placed in contact with a lens of focal length 10 cm so that the power of the combination becomes 5 D.

Q.16. A convex lens of power 3 D is held in contact with a concave lens of power – 1 D. A parallel beam of light is made to fall on the combination. At what distance from the combination will the bean get focused?

Q.17. A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with one another.
a). What is the power of the combination?

b). What is the focal length of the combination?
c). Is this combination converging or diverging?

Q.18. The power of a combination of two lenses X and Y is 5 D. If the focal length of lens X be 15 cm, then
a). calculate the focal length of lens Y.

b). State the nature of the lens Y.

Q.19. Two lenses A and B have focal lengths of +20 cm and – 10 cm, respectively.
a). What is the nature of lens A and lens B?

b). What is the power of lens A and lens B?

What is the power of the combination if lenses A and B are held close together?

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