In an experiment with a spherical mirror, a student measures the object distance $u$ and the image distance $v$ for several real objects. She follows the NCERT sign convention, with distances measured from the pole and positive along the direction of the incident light. She plots $\dfrac{1}{v}$ against $\dfrac{1}{u}$, both in $\text{cm}^{-1}$, and gets the straight line through the three points in the graph shown in the figure. The line does not reach either axis. What kind of mirror is it, and what is its focal length?

Fig. 1: A graph of 1 over v on the vertical axis against 1 over u on the horizontal axis, both in reciprocal centimetres, with gridlines every 0.05. Three dots are joined by a straight line that slopes down towards the right, in the upper left part of the graph. The dots are at 1 over u equal to minus 0.100, minus 0.050 and minus 0.020, with 1 over v equal to 0.150, 0.100 and 0.070. The line stops at the first and last dots and does not touch either axis.