What is the power of a convex lens with a focal length of $50 \ cm$?

$A$ ray of light $XY$ is incident on a spherical mirror as shown in the diagram. Redraw the diagram and mark the normal,angle of incidence and angle of reflection.

Show that the mirror formula for spherical mirrors also holds for a plane mirror.

The magnification in a plane mirror is always ........

An object,$4.0 \, cm$ in size,is placed $25.0 \, cm$ in front of a concave mirror of focal length $15.0 \, cm$. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and the size of the image.

What is the unit of refractive index?