Q29 of 84 Page 322

An object placed 45 cm from a lens forms an image on a screen place 90 cm on the other side of the lens. Identify the type of the lens and find its focal length.

According to the question;

Object distance, u= -45cm;


Image distance, v= 90 cm;


Focal length, f=?


By lens formula;







⇒ f = 30cm.


Since the focal length is positive, therefore the lens is convex lens of focal length 30cm.


More from this chapter

All 84 →
27

An object of size 7.0 cm is placed at a distance of 27 cm in front of concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed so that a sharply focused image can be obtained? Find the size and the nature of the image.

 28

A 5.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm By calculation determine

(i) the position and


(ii) the size of the image formed.

 30

Does the incident and emergent ray coincide in the process of refraction through glass slab? Give reason.

 31

A ray of light enters a diamond from air. If the refractive index of diamond is 2.42, what % (percentage) is speed of light in diamond with that of speed in air?