What is meant by power of a lens? Define its SI 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? Draw a ray diagram to justify your answer.
The power of a lens is defined as the reciprocal of its focal length. It is denoted using the letter P.
Mathematically,
;
where f is the focal length of the lens.
Given:
Focal length of the lens A, FA = +10 cm = +0.1 m
Focal length of lens B, FB = -10 cm = -0.1 m
Power of lens A, P = 
P = +10 D
Since, power is positive, the lens is a converging lens or convex lens.
Power of lens B, 
i.e., P = 
Therefore, P = -10 D
Since, power is negative, it is a diverging lens or concave lens.
Given, that the object is placed 8 cm from the lens, which is at a distance less than 10 cm.
In a convex lens, when the object is placed between the pole and focus, image formed is always virtual and diminished.
Therefore, the lens will produce a virtual and magnified image of the object.
Ray diagram is as shown below:
