Answer
See below.
Work Step by Step
Electric eye: a beam of light continuously shines on a light-detecting surface. Some of these "photodetectors" rely on the photoelectric effect - as long as the light shines, electrons are ejected and the resulting small current is amplified. When someone walks through the beam of light, the small current stops. This triggers some action.
Light meter: The photoelectric current varies, as the amount of light falling on the meter changes.
Sound track: A light continuously shines through the audio track on a piece of fast-moving film, which has some opaque parts and some clear parts. The light is transmitted, in a time-varying pattern, onto a photodetector. The photoelectric current is amplified and drives a loudspeaker.
