From lightning, we'll move onto the laser harp. In this case, the laser doesn't serve as the mechanism that produces the sound, but rather as the human interface device for playing the instrument.
The laser harp is connected to a device that will interpret the commands and play the sound - generally a synthesizer or computer. Lasers emulate harp strings, fanning out from a laser emitter, and when one of the laser beams is interrupted, a note is played. While some laser harps keep things as simple as that - block a beam with your hand to play a note - some measure the position of your hand to allow you to modify the pitch or even simulate the vibration effect of an actual stringed instrument.
How does the laser harp know whether you're blocking a laser beam, and how far along the beam your hand is? By using either a photodiode or a USB camera. The laser harp doesn't actually recognize that the laser beam is being broken - rather, it looks for the light that the laser beam would produce on a hand interrupting it. The USB camera is self-explanatory, but the photodiode is a strip of semiconductive material angled behind the laser emitter such that it will detect the telltale bright spot of a laser beam striking a hand. When the reflected light of the laser on the hand bounces back and strikes the photodiode, the photons are converted into an electrical current that signals that a note is being played.
But the photodiode isn't a camera - it doesn't know which note is being played, only that some note is being played. It's just a photon receiver. So how does the harp know which note to play? It's all about timing. While it may appear that all the lasers are being emitted simultaneously, they are, in fact, each being turned on and off at specific times. Too fast for the naked eye to perceive, the strobing effect is caught by the sensitive photodiode and the controlling device. This means that when the photodiode says "Hey! I got something!" there is only one possible laser beam that can be "on" at the precise moment in time.