how we use polarized light or polarizing microscopes
for studying biological subjects,
especially living cells.
And polarizing microscopes
have been used by crystallographers
and mineralogists for many many years
for looking at crystals and natural minerals.
But what we need for biology
is slightly different from what the
standard commercial polarizing microscope is.
And therefore, I'm going to make some demonstrations here
to explain how the polarized light microscope works.
And these demonstrations partly use crystalline material
and partly biological examples.
So what we have here is the word "birefringence,"
I hope you can see on your screen,
and what I'm going to do is,
in front of this word,
I'm going to place a crystal of calcite
Calcite is Iceland spar,
and Iceland spar is a crystal calcium carbonate.
And it's well-known because it introduces
birefringence, or double refraction.
Double refraction is the property
that we use very often in biology.
And one of the properties of this double refraction
is that the light that comes through the crystal
before it's just a single image becomes double.
And because it's double, it's called birefringence.
And now if we put a pair of sunglasses in front of it.
one or the other of the image disappears.
What this is telling us is that light can come through
or the birefringent material itself is being polarized.
Now I'll explain polarization in a minute.
But in any event, birefringence,
or birefringent material takes ordinary light
and then splits it into two
and that split light itself becomes polarized.
So now what we're going to do
is take two birefringent materials,
two polarizing materials, here is polarizing material,
and putting it under the birefringent
Under the crystal, and then depending
on how I set the crystal,
we should be able to see two images.
Now then, when I put a second polarizing material above
and cross the two,
then we'll still see the birefringent material.
As I turn the crystal,
then the image becomes brighter or darker.
And this is the property of birefringence.
Calcite is not only interesting as a special mineral
it has strong birefringence,
but also it occurs in many of our body's structures.
And for example, in bone, and teeth, and so on,
one of the mineral components is calcite.
And when we look at a sea urchin embryo
for example, we see even in the young embryo
tiny little specules which are made of pure calcite
and those show as birefringence.
Instead of the calcite crystal
I'm going to put in a birefringent biological sample here
which you may just be able to see or not be able to see
because the contrast is terribly weak between cross polarizers.
Now I'm going to put another birefringent material
and it should become brighter or darker.
So this is a model of a mitotic spindle,
which is weakly birefringent
in fact the birefringence is only a few hundredths
or thousandths of the birefringence of the calcite crystal you saw.
So we need this compensator
in order to see which way the molecules are lined up.
But the birefringent material itself is just like the calcite crystal,
separating,depending on the orientation of the material.
What the birefringence is showing us
is that molecules are lined up and what I'm doing
is taking this piece of plastic and pulling it so that
the molecules line up.
It becomes bright and even shows color
and then this is birefringent just like the other material.
But let's just see here
lining up the molecule
made this birefringent material change
into a birefringent material.
I'll demonstrate the same effect by using sound waves.
Although sound is still somewhat different from light waves.
I cut a piece of wood here
so that one block is running parallel to the grain
and the other is across the grain.
And sound travels must faster along the grain
than across the grain,
so when I tap on the edge of this
then you'll find the resonance sound.
This is parallel to the grain (fast tapping sound),
and this is across the grain (slow tapping sound).
And you see there's almost an octave difference
because sound travels twice as fast
along the grain than across the grain.
So this is acoustic anisotropy,
somewhat similar to optical anisotropy.
Except there is a fundamental difference
between sound and light.
Sound travels faster in denser medium,
whereas light travels slower in denser medium.
So this distinction you should remember.
But anyhow, this is double refraction of a kind.