If the radius of curvature of a concave mirror is $36\, cm$,what is its focal length (in $, cm$)?

Where should the object be placed to obtain a real and diminished image using a concave mirror?

What are the units of refractive index?

If a light ray $IM$ is incident on the surface $AB$ of a rectangular glass slab as shown,identify the correct emergent ray.

$(a)$ Define radius of curvature and focal length of a spherical mirror and show it on a figure.
$(b)$ Write the relation between the radius of curvature and the focal length of a spherical mirror.

$A$ ray of light is incident on a plane mirror normally. What are the angle of incidence and the angle of reflection?