How do photodiodes work?

Other Hardware Sensors
1

I'm making a laser harp and using photodiodes to detect the breaking of the laser beam... I'm going to detect Arduino as a MIDI instrument and connect it to Raspberry Pi that runs some sort of MIDI software to synthesize sounds generated by breaking the laser beam.

I would like to understand what happens when laser beam is broken? What do photodiodes do? How does Arduino know what to do? And how is the sound generated?

Thank you!

2
  1. Photodiodes works as Solar panel.
  2. It works as photo cell too.