An object 20 cm from a spherical mirror gives rise to a virtual image 15 cm behind the mirror. Determine the magnification of the image and the type of mirror used.

Given:

Distance of the object, $u$ = $-$20 cm

Distance of the virtual image, $v$ = 15 cm

To find: Magnification, $m$.

Solution:

From the magnification formula, we know that-

$m=-\frac {v}{u}$

Substituting the given values we get-

$m=-\frac {(-20)}{15}$

$m=\frac {4}{3}$

$m=+0.75$

Thus, the magnification is $0.75$ which is less than 1, which means the image is smaller in size, and the positive sign implies that the image is virtual and erect.

Hence, the mirror used here is convex, as it forms a smaller virtual and erect image.