Hard Light Surfaces 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 Thermal Discouragement Beams and excursion funnels). They can be used as floors or ceilings, block LOS from turrets or stop flying objects in mid-air, like cubes.
They can be painted with any kind of gel, however only repulsion and propulsion gel change anything. Portal shots and lasers can pass through hard light bridges or walls, which is why conversion gel and reflection gel don't work. Painting one side of the surface affects both sides.
projected_wall_entity, the actual entity representing the wall itself.
Example map with a VMF included.