Fotodiox Dedicated (Bayonet) Lens Hood, for Canon EF 85mm f/1.8 USM, 100mm f/2.0 USM for $6.95

Lens hoods are primarily designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact. Dedicated lens hoods are designed to match the specific focal length of the lens it was designed for.