We are all familiar with the motion of particles
from our every-day experience with solid objects. Like balls on a billiards
table, when two particles run into each other, they bounce apart. In contrast,
when two waves meet, they pass right through each other!
As the animation shows, when two waves overlap
they add up to form a larger wave
They can also cancel
out to form nothing at all (cancellation), or partially cancel to make
a more complex waveform. This is called interference.
When moving particles hit a gap in a barrier
they will generally trickle out in a thin stream, but waves will spread out
in every direction to form a circle. This is called dispersion.
If there are two gaps in the barrier, the two waves
will interfere to create an interference pattern.
For all these reasons, at the beginning
of the 20th century everybody thought particles and waves were
totally separate phenomena. But that view had to change.
Photons: the Particle Nature of Light
clipped from: www.physics.uiowa.edu 
