Question

what is meant by power of lens ? Define its S.I. unit.
u have two lenses A and B of focal length +10cm and -10cm respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8cm from the lens ? Draw a ray diagram to justify answer

Answer 1

.The power of a lens is defined as the reciprocal of its focal length. It is represented by

the letter P. The power P of a lens of focal length f is given as

P=1/f

The SI unit of power is dioptre (D).

Given:

Focal length of lens A, FA = +10 cm = +0.1 m

Focal length of lens B, FB = −10 cm = −0.1 m

In a convex lens, when the object is placed between the pole and focus, the image formed is always virtual and magnified. On the other hand, a concave lens produces virtual, erect but diminished image.Here the object is placed 8 cm from the lens which is at a distance less than the focal length, i.e. less than 10 cm. Thus, the 8 cm position of the object placed in front of the convex lens will produce a virtual and magnified image