Question

Magnification produced by a concave mirror is:
a. Is always less than one
b. Is always more than one
c. Always equal to one
d. May less than or equal to or greater than one

Answer 1

Magnification is defined as the ratio of the size of the image formed by the spherical mirror to the size of the object.Generally magnification is denoted by $m$. Magnification could be positive or negative depending upon the position of the object and the position of the image formed and height measured as negative for downwards and positive for upwards.

Formula for magnification:
$m = \dfrac{I}{O}$, where height of the image is $I$ and height of the object $O$.

Complete answer:
Magnification of the concave mirror depends on the position and erect and inversion of the image and object. We’ll try to understand the concept using two diagrams which can explain us better.

The diagram shows the image formation by concave mirror when the object is placed beyond the centre of curvature C. In this figure the object is AB and the image formed is A’B’. We can observe that the image formed is inverted in nature,therefore the size of the image is taken as negative because of inversion and the size of the object as positive because it is above the principal axis.
So, magnification is:
$m = \dfrac{{ \pm I}}{{ \pm O}}$, sign of the image and object size is based on erection and inversion.

Hence, the correct answer is option (d).

Note: Formula for magnification can also be given as:
$m = \dfrac{f}{{f - u}}$, where f is the focal length and u is the distance of the object measured from pole P.

Some of the uses of concave mirror are:
Used as reflectors in cinema projectors, used to construct reflecting type astronomical telescopes,as parabolic mirrors in search lights.