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 LOS from turrets or stop flying objects in mid-air, like cubes. Both can be painted with any kind of gel, however only Blue Gel and Orange gel will work properly. Portal shots and lasers can pass through hard light bridges or walls, which explains why conversion gel and reflection gel don't work properly. Painting one side of the bridge or wall will affect the other side.
Example map using this tutorial. VMF included.