Question

1) We are not able to see our face in a cardboard but ,can see in plane mirror why?

2) A person uses a concave mirror while applying make-up on his/her face.

Answer 1

1. Because cardboard don't allow light to reflect back from it's surface but mirror allows

2.
Concave mirrors magnify objects in focus. They're used for shaving or applying makeup because they reflect 'real,' almost 3-dimensional images. Concave mirrors are curved, and as light hits the upper and lower portions of the mirror, the image is focused at a central point, projected outward from the mirror itself. This means you do not have to look into the mirror itself to see your reflection, and the image is clearer and sharper than with a flat mirror.