Light Bridges are a very basic entity. They consist of a flat beam of solid light that the player can redirect through portals. They can be aimed in any of the 4 directions or enabled/disabled via input. They can also be coated with Gels, allowing for additional opportunities for puzzles. In Mari0 1.6, they were bright blue, while in Mari0 SE, they are more pale, with a subtle pulsating glow effect.
Light Bridges have many uses. They can help the player pass over troubled waters, or be used to block enemies, turning them around. Cubes can be placed on them, to be dropped on a button, or block a laser. Gels can also be splattered onto them, allowing for many unique puzzles to be created, especially in co-op play. They can be triggered using the linking system to create doorways or floors that fall out from beneath the player. In fact, two Light Bridges side-by-side are a perfect fit for Small Mario.
The emitter itself is not solid, though the beam is. This means that the effect is only noticeable if the beam is off.