Question

a student uses a lens of focal length 40cm and another of _20cm.write the nature and power of each lens

Answer 1

Hey there
 f = 40cm  so lens is convex
f = -20cm so lens in concave

To find  power use

P = 1/f Formula

Answer 2

F1=40cm

F2=-20cm

from lens formula;

1.P=1/F1

=100/40

power=+2.5 Dioptre(D)

Nature of lens=Converging lens or convex lens.

2.P=1/F2

=100/-20

power=-5 Dioptre(D)

Nature of lens=Diverging lens or Concave lens.