There?s a new thing that light does
that we introduced a few slides ago which is called diffraction.
When any wave impact on opening and it has to go through this opening
instead of staying in a straight line,
it will do this other thing called diffraction.
It will bend out in a sort of circle
and create this spherical shape on the other side of the opening.
You might ask me, well, this doesn?t seem to happen
if we have ocean waves and they come into a very, very big opening,
maybe from one city to the next, I don?t see it bending or anything like that.
And the reason is that this diffraction only occurs on any noticeable scale
if the width of the opening is comparable to the width
of the wavelength of the light that is impacting it.
So for light waves, for example,
if we have a very, very thin or very small wavelength of light
we would only really experience or see this diffraction effect
if the openings that were sending that light through
also has that very small wavelength.
We have a way of thinking about why this happens.
It seems like a pretty counterintuitive phenomenon
that you have something moving forward
and then instead of going forward and continuing on its path.
It takes this bended route.
There is actually theorems describing this
but the basic idea is that we imagine that as the wave goes forward
it's creating in fact different spherical waves
from each point along its path that is experiencing the wave.
So for example in this case,
as the light goes forward and tries to cross this boundary,
we can zoom in and see what we have shown here
which are different little molecules or different things
that taken the light and send back out the light.
And each one of these objects,
these small objects, send out light in a spherical pattern.
And so if we have a roll of these sending out light in a spherical pattern,
it looks just like a linear wave as it keeps propagating.
But if that pattern of atoms or molecules
or whatever it is that?s absorbing and readmitting the light is broken
as it is here since we have a barrier,
that light when it is sent out in a spherical pattern
will look like it bend around that barrier.
So again there?s theorems to describe in ways to understand
why this diffraction effect is occurring
where waves will bend around openings in some sort of a medium.
We can also use this effect to some sort of a practical purpose.
We call this shape here where we have many, many, many openings.
Again, each opening being somewhat comparable
to the wavelength of the light that we're sending through it.
We call this a diffraction grating
because we have this grating pattern of open and close and open and close
and we're using it to cause a diffraction effect.
We use a grating like this
to actually examine the different wavelengths of light
as it goes through the grating
since each wavelength will respond to this barrier differently.
We see this in many examples or anytime you have different openings,
different little, again very small openings for light to go through.
It will experience this diffraction
and cause the light to bend and react differently
allowing us to examine all of these different wavelengths separately
from each other.
There is one last and very interesting and very useful application
that we could use for this diffraction effect
which is called x-ray diffraction.
And it looks something like this,
suppose we have some material that?s in this blue box here,
and in this material we have many atoms for example
or many molecules all in some sort of a structure
and that?s represented by the small blue circles here.
So if it has some small, tiny structure like this
and we know diffraction occurs if we send in light
whose wavelength is comparable to the size between those structures,
then we know the light will diffract and bend through the material.
The question is, what wavelength of light do we need
to go through this material so that the light will bend and diffract
through these very, very small openings between these atoms and molecules?
It turns out that the wavelength is x-ray wavelength
or the frequency of x-rays,
so we send these x-rays in, they diffract through the material
and we examine the light that comes out the other side.
So this is a way of looking inside the material
without actually having to open it up.
We send light through,
it diffracts through all the different properties and materials inside
and then it comes out the other side, and that?s what we examine.
By just looking at the interference pattern or the diffraction pattern
that?s created on the other side of the material.
We can infer certain properties about the material
just by knowing the mathematics of how light bends
as it goes through the material.
So this diffracted light is examined
and again compared with some mathematical theorems
to determine the structure of the material itself.
Again, without having to actually open it up
or look at it or even use a microscope.
This very useful effect is interestingly applied to something like this,
this is an x-ray diffraction pattern
the light that comes out the other side of some soil form Mars.
So if we wanna know what kind material is on Mars.
Again, we don?t have to look at all the chemical composition
under a microscope
which would be very difficult to do.
We can instead just send x-rays though it.
Look at the pattern that?s created on the other side
which usually has this very sort of interesting
and symmetrical and bright and dark bands, looks quite nice.
And then we could just look at this pattern
and infer properties of the materials
and even determine what materials are present
just by again looking at the diffraction pattern
that we see coming out the other side.
So this finishes our basic introduction to light as a wave phenomenon
and some of the properties that light exhibits as it goes through materials.
So I am going to carry this on and see a few more properties of light
and a few new ways to think about light
as a wave and the particle in the coming lectures.
Thanks for listening.