A closed door feels absolute. Light stays in one room, darkness settles in the next, and the boundary seems obvious enough to stop thinking about. Yet the same wall that blocks the glow from your kitchen barely slows the Wi-Fi signal drifting through it. That mismatch feels strange for a reason. Visible light and radio waves belong to the same family. They are both electromagnetic waves. They follow the same basic physics. James Clerk Maxwell tied that picture together in the 19th century, and nothing in modern physics has overturned it. So the real puzzle is not why Wi-Fi moves through walls. It is why a wall treats two versions of the same phenomenon so differently. The answer has less to do with the wall being a barrier and more to do with the wall acting like a very selective filter. A popular explanation makes this sound simpler than it is. Radio waves have long wavelengths, people say, so they somehow slip through. Visible light has shorter wavelengths, so it gets blocked. That picture feels …