$13.3.5$ On the optical axis $AB$ of the collecting lens there is a flat mirror rotating with an angular velocity $\omega$ around the axis passing through point $A$ and perpendicular to the drawing plane. A parallel beam of rays falls on the mirror, which after reflection focuses on the screen. The focal length of the lens is $f$. Find the speed of the light spot on the screen at the moment when it crosses the optical axis. The screen plane is perpendicular to the optical axis.
