Hi. It’s Mr. Andersen and this is AP Physics essentials video 110. It is on wave diffraction,
which is the bending of waves as they move around an obstacle or through an opening.
And so you can imagine these are water waves here moving from left to right. And this is
an obstacle put in their path. And so as these ones on the top they just keep going and going
and going. But these ones right here are being diffracted. They are slightly bending around
that obstacle. If we look down here, as they move through an opening, diffraction occurs
as well. We are seeing that diffraction of these waves on this side. But there is not
a lot of diffraction going on. And the reason why is the obstacle is much bigger than the
wave length. What happens if we make that obstacle or that opening the size of the wavelength
or smaller? #00:00:51-9#
We get true diffraction. So what is happening is we are bending. And we get these concentric
circles that are moving out from there. Now wave diffraction is going to occur if we have
an opening for water waves like this, but always can be diffracted even light. So if
we put diffraction gradings, small slits, we can actually diffract light and break it
into its different constituent parts. #00:01:13-0#
And so wave diffraction occurs when we have either an obstacle or we have some kind of
an opening. And as the waves hit the obstacle they will bend slightly around it. So an obstacle
can diffract waves and also a slit. So as it moves through we are going to see a slight
amount of diffraction as well. It is bending around that opening. But what happens if we
take that slit and we make it the size of lambda, the size of the wavelength? What you
get is true diffraction that is going to look like that. So what is an example of this?
Let’s say that you are here and somebody is talking in another room, can you hear them
talking? Yeah. And that is because the sound waves are relatively large. As they move through
the door they are going to be diffracted. So you can hear what they are saying. It might
not be super loud but you can still hear it. Now can you see them around the corner if
you are in a different room. No. Even though light can be diffracted, look at our opening.
Our opening is so big compared to the wave of the light that it is not bending around
that corner. #00:02:14-6#
Good way to see wave diffraction is to use a ripple tank. And so what you do is put a
table that has water in it. You then have a motor that just goes up and down. And as
it goes up and down, this board oscillates and that creates waves that move out. You
can shine light through it. And then we could take a picture on the bottom. And so this
is what it would look like here where you have these waves that are being created. And
then you could see this gap. So they are moving right through this gap right here. And so
you would see diffraction on the other side. Now the size of that gap is important. If
you have a gap that is really really large in relation to the wave length, small diffraction.
But if we decrease it to around the size of the wave length then we are going to get true
diffraction. #00:02:55-4#
Here is a little PHET simulation of that. So we are generating some waves on the left
side, water waves and we are seeing diffraction over here. You can see it bend around. But
watch what happens as we make that smaller and smaller and smaller, look at that. We
are getting more and more diffraction around that obstacle. Same thing with an opening.
We do not have much diffraction on the edges of that. But watch what happens as we decrease
the size and then we get it close to a wavelength. Then we are starting to get true diffraction.
So this occurs for water waves. This could be a speaker on the left that is generating
sounds waves. You can see we have diffraction of those sounds waves as they move through
that opening. And we would also find that we are going to have wave diffraction if we
are looking at light. So we are generating light. If we can make that opening so it is
around the size of the wavelength of the light, again with light it is going to have to be
really, really small, then we can get diffraction. We could even see it in the wild. So we have
right here waves coming in on the ocean. There is a little opening right here and you can
see that diffraction as it moves through that little gap in the water. And so did you learn
to make claims about diffraction pattern. That is the pattern created as we move through
a small opening? Again we want to match the wavelength. And I hope that was helpful.