The light source may be a small 12 volt light bulb inside a fruit juice can or a spice can.  We may place aluminum foil over the end of the can and poke a small hole in the foil with a pin or we may hold two utility knife blades to the end of the can with rubber bands to form a slit.  Note that in this drawing the distance from the mirror to the focal area has been shortened to make the drawing more clear.

 

Setting up the light source