Question

a student uses a lens of focal lenght 40cm and another -20cm write its nature and power of each lens

Answer 1

Power of focal length 40 cm = 0.4m
p = 1/f
p = 1/0.4
p = 10/4
p = 2.5 D
It is a convex lens
Power of focal length -20 cm = -0.2m
p = 1/f
p = 1/-0.2
p = 10/-2
p = -5 D
It is a concave lens

Answer 2

those lens whose focal length is 40 whose nature is convex lens

and those lens whose focal length is - 20 whose nature is concave lens