$A$ virtual image three times the size of the object is obtained with a concave mirror of radius of curvature $36 \ cm$. The distance of the object from the mirror is ........ $cm$.

$A$ straight metal rod of length $6 \ cm$ is placed along the principal axis of a concave mirror of focal length $9 \ cm$ such that the end of the rod closer to the mirror is at a distance of $15 \ cm$ from the pole of the mirror. The length of the image of the rod is (in $cm$)

While capturing solar energy for commercial purposes,we use:

In an experiment to find the focal length of a concave mirror,a graph is drawn between the magnitudes of $u$ and $v$. The graph looks like:

When an object is placed $40\ cm$ away from a spherical mirror,an image of magnification $\frac{1}{2}$ is produced. To obtain an image with magnification of $\frac{1}{3}$,the object is to be moved $:$

The focal length of a convex mirror is $20\, cm$. Its radius of curvature will be.....$cm$.