Which device uses a beam of light to measure the speed of a motor's shaft?

The device that uses a beam of light to measure the speed of a motor's shaft is the photo tachometer. This instrument operates by directing a light beam onto a reflective surface, often attached to the rotating shaft of the motor. As the motor spins, the reflection of the beam back to the photo sensor occurs at a rate that corresponds to the rotational speed. By analyzing the frequency of the reflected light pulses, the photo tachometer can accurately calculate the shaft's speed, providing real-time feedback on motor performance.

Other devices listed, like oscilloscopes, frequency meters, and digital multimeters, serve different functions in electronics and measurement. An oscilloscope is used primarily to visualize electrical signals over time, a frequency meter measures the frequency of electrical signals, and a digital multimeter is a versatile tool used to measure voltage, current, and resistance. None of these devices specifically measure the speed of a rotating shaft using a light beam, which is why the photo tachometer is the correct answer.