What type of device emits light as a signal when current passes through it?

The correct answer is a diode. A diode is a semiconductor device that emits light when an electrical current passes through it, particularly when it is a light-emitting diode (LED). In simple terms, when electrons and holes recombine in the semiconductor material of the diode, they release energy in the form of light. This property is harnessed in various applications, ranging from indicator lights to sophisticated displays.

While an indicator may use a diode to show that an electrical current is flowing, it is not itself a device that emits light directly. Instead, it typically refers to a complete assembly or a circuit that may include a diode. Transistors and capacitors do not emit light; transistors are used for switching or amplifying electrical signals, and capacitors are used to store electrical energy. Thus, understanding the unique function of a diode provides clarity on why it is the correct answer for a device that emits light as a signal when current flows through it.