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

Explanation

Power of a lens and its S.I. unit . The power of a lens is a measure of the degree of con-
reciprocal of its focal length rays falling on it. The power of a lens is defined as the
reciprocal of its focal length in metres.
Power of a Lens = `(1)/(" Focal length (in m)")`
Lens A: `F_(A) = + 10 cm = (+10)/(100) = 0.1m, P_(P) = (1)/(F_(A)) = (1)/(+0.1) = + 10 D`
The power of a convex lens is positive therefore, lens A is a conbex lens.
Lens B: `F_(B) = -10 cm = (-10)/(100) = - 0.1m, P_(B) = (1)/(F_(A)) = (1)/(-0.1) = - 10 D`
The power of a concave lens is negative
therefore, lens . What an
object is placed at 8 cm (i.e., between the optical
centre and principal foucs) 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 wetween the optical centre and the focus: (i.e., between O and F') the image formed is behined the object (on the same side), virtual, erect and magnified.