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

Where should an object be placed in front of a convex lens to obtain an image on the same side as the object?

Two adjacent walls and the ceiling of a rectangular room are mirror surfaced. The number of images of himself that an observer sees is

Which of the following can produce a magnification of $+0.5$?

$A$ convex mirror always forms a virtual and erect image.

Define the following terms in relation to a concave spherical mirror (any five):
$(i)$ Pole
$(ii)$ Centre of curvature
$(iii)$ Radius of curvature
$(iv)$ Principal axis
$(v)$ Principal focus
$(vi)$ Aperture
$(vii)$ Focal length