Question: What is a light pen?

Answer:

A light pen is an input device that was commonly used with early computer systems and graphical user interfaces (GUIs). It is a handheld device with a light-sensitive tip or sensor at one end and a cord connected to the computer at the other end.

When using a light pen, the user would interact with the computer screen by touching the pen's tip to the surface of the display. The light pen detects the screen's electron beam as it scans across the display, allowing the computer to determine the position where the pen was touched.