Hard Light Bridges and Hard Light Walls are a puzzle element of Portal 2. They are solid slats of light produced by special emitters, and can be redirected through portals (just like lasers and funnels). Bridges are horizontal, and players can use them as a floor or ceiling. Walls are vertical, and can be used to block turret fire or catch flying objects, like cubes. Both can be painted with any kind of gel, however only Blue Gel and Orange gel will work properly. Painting one side of the bridge or wall will similarly effect the other side.
Hard light surfaces are emitted by a single prop_wall_projector.
Example map using this tutorial. VMF included.