Total Internal Reflection used in an Optical Mouse

The piece of plastic pictured below has been removed from an optical mouse. This clear plastic insert has a small vertical lens that collimates light from a LED. The path of the light is changed twice by total internal reflection and finally spread onto the surface that the mouse is placed on.

The optical sensor chip in the mouse shown below is positioned above the plastic insert’s horizontally mounted lens and takes images of the surface light pattern about 1500 times a second.

On this last photo I have drawn the approximate path of light rays from the LED through the plastic.