What are bicycle reflectors made of?

If you take one apart, you'll see that the reverse side of the plastic lens (the clear or red part) has been molded with hundreds of tiny pyramidal shapes. When light enters the reflector through the plastic, the angled sides of the pyramids act as tiny mirrors to reflect the light back out in different directions.

Actually the question asks how it is MADE. They are injection molded, you can tell this from the ejection pin marks found on the inside (or outside) surface. They are usually made from PMMA (acrylic).