Difference between mechanical mouse and optical …

A mechanical mouse has a ball that turns rollers inside. If friction is lost between the ball and the mousing surface, or between the ball and the rollers, the mouse fails to work. In order to assure good contact with the mousing surface, the ball must be fairly heavy. When you change directions with the mouse, you must make the ball change rolling directions--an action that inertia likes to prevent.
An optical mouse makes use of an LED and some optics to detect surface texture and the changes in it as the mouse is moved. There are no moving parts. My optical mouse works on all the surfaces I've tried it on except the white plastic fold-up tables. For those, a piece of paper serves as a suitable "mouse pad".