Question

you are given for spherical lens to convex lens of focal length 40cm 100 cm respectively and two concave lens of length focal length 40cm 100 cm respectively out of these two which one is suitable to be used as a magnifying glass and why​

Answer 1

A concave lens always produces a diminished image so, it can not be used as a magnifying lens.

Let us calculate the power of the convex lenses.

First lens:

f = 40 cm = 40/100 m = 2/5 m

therefore, P = 5/2 D = 2.5 D

Second lens:

f = 100cm = 1m

therefore, P = 1 D

The convex lens of focal length 40 cm should be used as a magnifying lens because it has more power.

More power means more convergence of light rays, which means the image will be magnified.