What is a primary application of a phototransistor?

A phototransistor is specifically designed to respond to light. Its primary application revolves around its ability to detect and respond to varying light levels. It does so by generating a current or voltage output proportional to the light intensity incident on its light-sensitive junction.

This functionality makes phototransistors valuable in various applications such as light sensors, automatic lighting systems, and optical communication. The device can switch or amplify signals in response to light, thus integrating seamlessly with circuits that require illumination detection.

The other choices pertain to functions that do not align with the primary operation of a phototransistor. Amplifying audio signals is typical of audio amplifiers, while converting electrical energy to magnetic energy and storing electrical energy are functions associated with inductors and capacitors, respectively. These distinctions highlight why detecting light levels is the principal role of a phototransistor.