B+W 55mm #950 Screw-In Metal Lens Hood
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.
The B+W #950 lens hood is rigid and made of black anodized aluminum. It is designed to match the focal length of standard lenses (50mm).