"There are more things in heaven and earth,
Horatio, than are dreamt of in your philosophy"
William Shakespeare (Hamlet)
One of the central predictions of quantum mechanics is that of "Wave-Particle
duality" . Practically, this means that under certain
conditions particles can display properties usually associated with
waves and waves can display properties usually associated with
particles. Confused ?!
If a wave exhibits particle properties then instead of the wave
appearing to be continuous (think of water waves) it behaves like
a stream
of particles (think of bullets leaving a machine gun).
If a particle exhibits wave properties then instead of being
localised (a baseball is only found in one place) it is "spread
out" like
a wave and consequently will have a definite wavelength.
But exactly what are the conditions we must satisfy for this
duality ? Clearly, in our daily experience waves are waves and
particles
are particles. Are you sure ? Think again.
We have already touched on a situation where waves behave as
particles. In our discussion of Geometric Optics (Reflection) we
assumed that visible light waves behaved as a stream of particles
(light travels in straight lines) so long as we don't look too closely
at edges and apertures.
Is light a wave or a particle ?
The relevant conditions are as follows:
Electromagnetic waves behave like particles at high energy
. There is no dividing line; all electromagnetic waves can
exhibit particle properties, they just become more obvious as the
energy of the wave increases. "Particles of light" are
called photons
.
Particles display wave properties when their mass is small
. Again, there is no dividing line; all particles have wave
properties, but only when their mass is small enough is it possible to
observe them.
a
Two
atoms bump into
each other. One says 'I think I
lost an electron!' The other asks, 'Are you sure?', to which the first
replies,
'I'm positive.'
