An object is placed at $20 \ cm$ in front of a concave mirror,which produces a three times magnified real image. What is the focal length of the concave mirror (in $cm$)?

$A$ concave mirror of focal length $100 \ cm$ is used to obtain the image of the sun,which subtends an angle of $30'$. The diameter of the image of the sun will be.....$cm$.

Assertion $(A):$ The relation among $u, v$ and $f$ for a spherical mirror is valid only for mirrors whose sizes are very small compared to their radii of curvature.
Reason $(R):$ The laws of reflection are strictly valid for plane surfaces but not for large spherical surfaces.

$A$ short linear object $O$ is placed on the principal axis of a convex mirror of focal length $f$ as shown in the figure. The ratio of transverse magnification to longitudinal magnification is

$A$ spherical mirror forms an erect image three times the linear size of the object. If the distance between the object and the image is $80 \, cm$,the focal length of the mirror is........ (in $, cm$)

$A$ concave mirror gives an image three times as large as the object placed at a distance of $20 \ cm$ from it. For the image to be real,the focal length should be ...... $cm$.