Canon ET-83II Lens Hood for Canon EF 70-200mm f/2.8L USM Lens - 2697A001
One of the most important accessories for each lens you own. A lens hood provides multiple functions: it shades the lens from stray light, improving your contrast and image quality; in inclement weather, it can assist in keeping moisture or wind-blown debris off the lens; and it protects the front barrel from the inevitable impacts against walls, door frames, and other real-life obstacles. Almost all EF and TS-E lenses have a Canon lens hood engineered specifically for it.
Amazon.com Product Description
Lens hoods are primarily designed to prevent unwanted stray light from entering by extending and shading the end of the lens, ensuring no problems with vignetting or motor functions. Additionally, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact. Lens hoods match the specific focal length of the lens it was designed for.