Question

you have to lenses a and b of focal length + 10 centimetre and minus 10 centimetre state the nature and power of each lenses which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens

Answer 1

+  10 centimeter focal length is a converging lens -  Convex lens

- 10 centimetre focal length is a diverging lens.  - Concave lens

The convex lens will give virtual and magnified image of an object placed 8cm from the lens.

Answer 2

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.