so the symbol i'm going to talk about is
a thing called birefringence but
actually what i'm going to show you
first is this beautiful crystal crystal
of calcite that is a big crystal it's an
enormous crystal it's actually just
calcium carbonate so it's the same stuff
that limestone and marble are made of
but just in some places in the world it
just forms into these perfect crystals
of calcite it's kind of also called
iceland spa because i think the first
first found in in iceland
so if you just focus on the square there
ready yeah okay now if i put the crystal
in there you can see that the image
instantly becomes doubled
it's a strange property of this
particular
type of crystal that it creates kind of
double images of things but there's
another even stranger property which is
that these are now the individual images
are each polarized and i can show you
that by sticking a sheet this is just a
regular sheet of polaroid and if i get
this just right so some angles we see
both images but as i rotate it around
sometimes you'll see one image
and as we keep going around sometimes
you'll see the other image
come back the other way again
there we go they fade between one image
and the other and that's because one of
the images the light is polarized in one
direction and the other image the light
is polarized in the other direction
i want to see if i can get your face to
double i don't know if it
yeah it doesn't work so well okay yes
we're
yeah you wanted to see the try to do the
thing with the polaroid as well and see
if we can do that okay
i just rotate it around
yeah that works cool you can explain it
okay
you need to sit down for this okay so
it's a thing called biorefringence and
it's to do with refraction uh which is
the property of material every material
has a refractive index and it's
basically to do with the speed of light
right the speed of light in a vacuum is
always the same 300 000 kilometers per
second but the speed of light through
different materials is different um that
basically the speed of light slows down
in different materials and essentially
it's because as the lights traveling
through lights this electromagnetic wave
that's wiggling around the electric
fields are wiggling around that makes
the the electrons in the in the material
wiggle around as well and as they wiggle
around they produce electromagnetic
waves of their own which kind of combine
with the light that was already passing
through the net effect of all that is to
make the light as it travels through
slow down
okay and we use this all the time i mean
it's why uh lenses work why lenses bend
light and so on and the reason why so if
we have to go back to our crystal here
right so if we got a light approaching
this crystal
as it approaches of course if it's not
if it's face on then all the light hits
at the same time but if it's coming in
at an angle then some of the light hits
first and then the rest of the wave hits
later on so as this first bit of light
hits it slows down
but this bit up here that hasn't hit the
surface yet it's still going at the same
speed the net effect of that is like if
you were to drive through a puddle in
your car right only and the puddle only
hits one set of wheels what tends to
happen is the wheels on that side slow
down and you'll find your car tends to
try and turn that way right so it's just
the same with light the light is here
the bit that hits first slows down and
the net effect of that is that it turns
the direction of the light
so one of the effects of glass of of
light hitting a surface like this where
there's a refractive index in this
material is it changes the direction in
which light is going okay that's this
effect called refraction and that's how
prisms work and lenses work and all
sorts of things like that
so
that's that's a well-known phenomenon
the interesting thing about this
particular material is it has different
refractive indices so it slows down the
light by different amounts depending on
what the polarization of light is so one
polarization of light gets slowed down
more than the other polarization of
light that means that if you've got one
wave coming in which is kind of made up
initially of both polarizations of light
one of those waves is going to get bent
more and one of them is going to get
bent a little less and that means that
as the light travels through the images
of something that you're looking at
split up because the the light gets
shifted by different amounts on by in
the two different polarizations so what
how is that possible then how come that
material does this magical thing with
light and
your window there doesn't it's to do
with the crystal structure of the glass
so this is a calcium carbonate crystal
calcium carbonate has this rather
particular structure i'll show you a
picture of it here so this is actually a
picture of calcium carbonate um i'm
getting scarily close to chemistry here
but basically it's made up of calcium
atoms carbon atoms and oxygen atoms in
these various different colours well
that's quite good for you chemistry-wise
that's quite important about my the sum
total of my knowledge but if you look at
so this is how they're actually arranged
within the the structure of the crystal
and what you can see is that there's a
kind of a particular access to this
whole thing there's all these little um
little red groups kind of arranged
sticking out to the sides and then uh
the the calcium atoms are kind of in
this more vertical structure and so you
can see there's kind of an axis that
runs up and down this particular thing
in this direction here and so you
wouldn't be terribly surprised if the
properties of this material so remember
what happens is a light wave goes
through so imagine i've got my light
wave jiggling electric field jiggling
backwards and forwards as it goes
through this way
okay as it goes through it's going to
shake these atoms in this direction and
shake their electrons in that direction
and because of the structure the very
strange structure of this thing it
wouldn't be too surprising to learn that
actually if you try and shake these
atoms in that direction they react
differently than if you try and shake
them in that direction and that the
microscopic level is the reason why the
refractive index of these two
polarizations of light one with the
electric field jiggling one way and one
with the electric field jiggling the
other way are different and that's why
the light propagates through in a
different way
and in fact since this is 60 symbols i
can even give you a symbol for this
it's called the birefringence and it's
actually called delta n because n is the
symbol that's usually used for
refractive index and what you're
measuring here is a difference in
refractive index so it's a delta for a
difference delta n you're seeing the
light so really you're seeing two images
because the light that came into this
thing and kind of was reflected off that
blue square and so the blue light was
reflected back to you one set of that
blue light followed one set of paths
through that crystal and the other set
of blue light followed another set of
paths and they end up offset from each
other so you see the square in different
places i mean the light that's that's
filling this room is basically
unpolarized but one of the things you
can do is you can always split light up
okay this light is kind of you know
randomly polarized but every little bit
of it
actually has a polarization at every
point in space the light is oscillating
one particular direction and we can
always break that an oscillation in that
direction into a little bit in that
direction a little bit in that direction
so you can always break it down into two
individual polarizations and that means
that this complete combination of
unpolarized light all these different
random oscillations of the light in all
sorts of different directions we can
always break down into two components
and then it's those two components that
are acting in these two different ways
when you actually end up looking at
through the crystal like it's it's x's
and that's why it's exercise exactly wow
the squares move wild what's going on
there mike that's just because you're
now looking through
in different directions relative to the
axes of the crystal and so by looking
through in those different directions
relative to the axis of the crystal
you're changing the refractive index of
the of the class
where can people get one of them
everyone's going to want one now go down
your gem and geology shop that quite
often yeah they have them this isn't big
the biggest one i've ever seen i used to
have a really little tiny one which
didn't look anywhere near as impressive