Q4 of 10 Page 107

What do you mean by power of a lens? What is the SI unit of the power of a lens? Calculate the power of a concave lens of focal length 25 cm.

Power term related to focal length of a lens. The reciprocal of the focal length of a lens in metres is called the power of the lens.

Its is a capacity of a lens to bend the rays of light. The smaller the focal length, the greater is the bending of a ray of light and vice- versa.


SI unit of power is diopter represented by D


Given:


concave lens of focal length =25 cm


From the formula of power,



Here focal length of concave length is taken as negative.


Therefore concave lens of focal length =





Power of concave lens is: - 4 D


More from this chapter

All 10 →
2

The nature of images formed by two lenses are given.

(i) An erect and magnified virtual image


(ii) An erect and diminished virtual image


On using which of these lenses will we get an image having the same size as the object? What is the position of the object?

 3


(a) MN represents a lens. What type of lens is this?


(b) What are the characteristics of the image?


(c) Copy the ray diagrams in the science diary and complete it.

 5

Observer the figure. Light falling on two different media are shown.


(a) Which medium has greater optical density?


(b) Why?


(c) Which medium has a greater refractive index?

 6

An object of height 3 cm is placed in front of a convex lens of focal length 10 cm at a distance of 15 cm.

What is the distance of the image formed?