Intro 
Quantum physics tells us that a quantum entity (Nick Herbert
calls this, generically, a quon) has a dual mode of existence: When unobserved it exists as
a wave of potentials (possibilities); when observed, the wave collapses into a particle
(e.g., photon, electron, quark). Wave function collapse is the event in which the particle appears in one of its many wave potential states/locations. 
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Ideas to Explore 

The Twoslit Experiment The Feynman Doubleslit Doubleslit Experiment 
Take your pick: Explanations with static and animated graphics. 

Lessons in Holography 
Photons, corkscrew waves, interference patterns, standing waves, holograms 