C899 Phantom Stairway!

By now, Fang Rui more or less knew why such energy-consuming infrared ray could exist here.

If it was always infrared, then the energy cost would undoubtedly be huge. However, this place's device was very smart, it could actually reuse this infrared light. This way, if nothing passed above, the infrared light would be self-sufficient, and it would only need a very weak energy to replenish itself