. What is meant by power of a lens Define its S.I. unit
You have two lenses A and B of focal lengths +10 cm and -10 cm 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 8 cm from the lens
Ans. Power of a lens and its S.I. unit.
The power of convex lens is positive therefore, lens A is a convex lens
The power of a concave lens is negative therefore, lens B is a concave lens.
When an object is placed at 8 cm (i.e. between the optical centre and principal focus) only convex lens will form the virtual and magnified image.
Therefore, lens A will form a virtual and magnified image of the object placed 8 cm from it. When the object is placed between the optical centre and the focus: (i.e., between 0 to B' and F') the image formed is behind the object (on the same side), virtual, erect and magnified.