Question

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

Answer 1

Power is defined as reciprocal of focal length.
P=1/f (m) 
S.I unit of power is Diopters or D.

lens A =Focal length =+10cms 
lens B=focal length = -10cm

Lens A is convex lens as focal length is positive and lens B is concave lens.

Convex lens will form virtual and magnified image when an object  is placed 8cm from the lens[That is when  the object is placed between P and F  ]
Please refer the attachment for diagram

Answer 2

Answer: Power of the lense is the ability of the lense to converge the rays of the light falling on it.

SI unit = Dioptre

It is to remember that focal length of convex lens are positive & focal length of concave lens are negative.

And convex lens will form virtual & magnified image when object is in b/w focus & pole of lens.