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Waves bend around corners (diffraction) and interfere with one another, producing interference patterns. If a wave encounters a small aperture, the wave spreads out on the other side as if the aperture were a point source. The energy of waves is spread out in space and deposited continuously as the wave fronts interact with matter.

Particles travel in straight lines until they collide with something, after which the particles again travel in straight lines. If two particle beams meet in space, they never produce an interference pattern. Particles exchange energy in collisions that occur at specific points in space and in time.

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Excerpted and adapted from: Tipler, Paul A., and Gene Mosca. 2008. Physics for Scientists and Engineers. W. H. Freeman and Company, NY. 6th Edition, p 1174.