What device produces a narrow beam of electrons in a cathode ray tube by thermionic emission?

The correct choice, the electron gun, is a crucial component in a cathode ray tube (CRT) that produces a narrow beam of electrons through the process known as thermionic emission. In thermionic emission, electrons are emitted from a heated cathode when the thermal energy provided by the heat exceeds the work function of the material, allowing the electrons to escape into a vacuum.

Once emitted, these electrons are focused and accelerated toward the screen of the CRT, forming a precise beam that can be steered and manipulated to create images. This process is essential in applications such as oscilloscopes and older television displays, where the electron gun plays a vital role in displaying visual information.

In contrast, a transistor primarily functions as a semiconductor device used for amplification or switching, and does not produce electron beams. An oscilloscope, while it utilizes a cathode ray tube and includes an electron gun as part of its operation, is not the device that directly produces the electron beam on its own. A photodetector is designed to detect light rather than generate electron beams. Thus, the electron gun is the specific device responsible for creating a narrow beam of electrons in CRTs through thermionic emission.