A proof-of-concept touchscreen MacBook was created with a dollar’s worth of hardware, called Project Sistine. The main principle behind Sistine is the surface reflection.
Surfaces viewed from an angle tend to look shiny, and you can tell if a finger is touching the surface by checking if it’s touching its own reflection.
The hardware is a modified version of ShinyTouch. By fitting a small mirror in front of the MacBook’s built-in webcam, the software can see fingers touching or hovering over the display. It’s a cool concept that you can read more about here.