Here's why:

* Optical: Light pens work by detecting light emitted from the screen. They have a light-sensitive element (often a photodiode) that converts the light into an electrical signal. This signal is then used to determine the pen's position on the screen.

* Not Mechanical: They don't rely on physical contact or movement like a mechanical pen. They don't directly touch the screen, instead relying on light to determine their position.

Let me know if you have any other questions about light pens or other input devices!