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Direction of Light Emission Within Solid
Name: Don
Status: educator
Grade: 9-12
Location: KS
Country: USA
Date: Summer 2011
Question:
Light is transmitted through glass with a photon moving
from one atom to the next. What determines the direction the photon
is emitted and seems to pass (straight line)? How does this differ
from scattering?
Replies:
I would differ with your implied description, Don.
During transmission through transparent materials,
photons are in not caught by the atoms and re-emitted.
It is much more like a sea-wave gliding through a field of pier-posts.
The mass and elastic looseness of the electron-clouds of the atoms
adds some slowing effect to the underlying EM wave medium (space itself).
But mostly it is the same momentum-like preservation of wave vector
and the same photons that keep on going.
Notice that the wavelength is much longer than both
the width of an atom and the separation between atoms.
This makes it very plausible to transmit without scattering.
The direction-determining values of the wave
are dispersed across a much larger volume than that of any one atom's cloud.
Those properties are: the direction of the magnetic field, and the direction of the electric field.
The axis perpendicular to both is the wave's direction of travel.
One atom can only change those directions within it's own small zone of influence,
and often they aren't changed in direction, just in strength.
So the wave's average vector directions remain little changed.
If photons were absorbed and re-emitted, it would be scattering as you surmised,
and it would be translucent not transparent,
because the direction of re-emission would not depend much on the direction of incidence.
In practice, re-emitted light-photons usually leave a substantial portion of their energy behind on the atom.
This is the case of fluorescent pigments, which for example may absorb blue and re-emit green or yellow.
Their re-emissions are indeed randomly isotropic (uniform in all directions).
Quite scattered as well as shifted.
There is a subtle effect called Raman scattering
which is very much between those two cases.
It is used in a scientific instrument which analyzes almost any transparent material.
This Raman scattering adds only a slight energy-loss/color-change,
and gives only a slight angle of scattering.
Both the angle-change and the color-change are used
to distinguish the weak Raman emission from the strong probing laser beam which goes right through.
Different materials yield differing amounts of color-shift ("Raman shift").
Jim Swenson
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Update: June 2012
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