Lenses or mirrors collect the light scattered by the microscopic particles, and the light is reflected onto a semiconductor photodetector. The photodetector turns the light energy into an electrical current that is proportional to the brightness of the light. Since the light was proportional to the size of the particle that scattered the light, the amplitude of this electrical pulse is also proportional to the size of the particle. We now have a device that not only detects the presence of something invisible to our eyes, we also have information about the relative size of those invisible particles.