Optical smoke sensor

With no small particles in the air, the IR beam goes straight into the wall of the housing. The wall is black and absorbs a great deal of the radiation. The little fins might be angled in such a way that the reflected radiation bounces around many times, which further attenuates it. Ideally nothing ever hits the sensor. The main purpose of this construction is to allow smoke-carrying air penetrate the walls, but keep ambient light out.

When tiny smoke particles are present in the center of the sensor, they scatter some of the IR light towards the sensor. You hear a very load beep.