An ordinary light sensor answers the question "how much light" but not "where did it come from." To do that, you need lenses, mirrors, or entire arrays. In a new study, researchers propose a sensor that determines the direction of a light beam on its own, without any optics.

Inside the sensor is a thin layer where electrons move freely, like water in a shallow dish. Two metal contacts are pressed against the edges of the layer. When light hits at an angle rather than straight down, the light wave enters the layer unevenly: it's stronger at one edge and weaker at the other. The electrons respond to this imbalance and start moving — creating an electric current. And the current flows even at zero voltage, all by itself.

To determine not just the side but the exact angle, you need to make the electrons oscillate in resonance. By changing their density (like tuning a guitar string), at a certain density the oscillations become especially strong. From how strong the current is at that point, you can calculate the angle of incidence.

Such a sensor doesn't need lenses, mirrors, or moving parts. It's flat and can be very small. In the future, similar devices could be useful in robots, navigation systems, and even augmented reality glasses, where knowing where light comes from is important.