dominated by waves. Right now you're listening to me and so you're picking up sound waves

from the speakers in your computer. Since you're watching me, that's electromagnetic

waves. If you're listening to this on wifi then you're using radio waves to pick up that

signal. So what is wave? A wave if we define it is simply a disturbance that moves through

space and time. It's a good way to take energy or information and move it from point A to

point B. So waves are really important. But there are a few properties that you need to

understand about waves before you really get it. First of all you should understand that

waves come in two different flavors. There's transverse waves and longitudinal waves. Transverse

waves, an example of that, if you were to tie a string to a tree and then just move

the string up and down you'd be creating a transverse wave. How does that work? Well

the string would be moving up and down. But the wave would actually be traveling perpendicular

to that. And since this is perpendicular motion we call that transverse wave. And this does

kind of look like a T on its side. And so that's a good way to remember what a transverse

wave is. A longitudinal wave, an example of that would be the sound you're listening to.

It doesn't oscillate perpendicular to the motion. It actually oscillates in the direction

of the motion. And so this video shows you some longitudinal waves. What's happening?

Well the oscillation is in this direction. And the motion is in this direction as well.

And so that's a longitudinal wave. It could be like this in water waves. Or it could be

air waves as well. But that's a longitudinal wave. There are some properties you should

understand about waves as well. And in fact there's a relationship that's worth memorizing.

And that is V equals f times lambda. What does that mean? Speed of a wave equals the

frequency of the wave times the wave length. We always measure speed in meters per second.

We measure frequency in hertz which is one divided by time. Or one divided by a period

or second. It's called a hertz. And then wavelength is going to be measure in meters. So let me

kind of go through these three properties of a wave. If you're looking at wave speed

it's easier to measure wave speed when you're just looking at one wave. And so let's say

for example that we're measuring a wave and we want to see how long it takes to move from

point A, we'll say over here, to point B which is over here. Well let's put the wave in motion.

Let me time it. One one thousand two one thousand three one thousand. So let's say it takes

3 seconds to move from point A to point B. And let's say that that's 3 meters to make

the math easy. Well it's now moving 3 meters in 3 seconds and so it would have a wave speed

of one meter per second. Another thing that's interesting to look at in this animation is

that the actual particles on the wave don't move as fast as the wave. The closer you get

to the surface, if you're a surfer, the faster you could move. But a lot of those particles

are barely moving at all. And so the energy is being travelled through the medium. But

the medium is not actually being travelled. Next thing is going to be called frequency.

Frequency is how often waves come. And so the definition for that is one wave divided

by T which stands for the period. In other words if we have one wave every one second,

then we would call that a frequency of one Hertz. So let me put this animation in and

run. So right here we've got a series of lights. So the light at the bottom, it's blinking

every 0.5 second. And so it's period is 0.5 seconds. And so it's 1 divided by 0.5 or 2

hertz. Let's say we have a wave that come this often, every 2 seconds. So we have a

wave. And then one one thousand, two one thousand, wave. So that would have a frequency of 0.5

hertz. In other words the faster the waves come the bigger the frequency is going to

be. The larger the frequency is going to be. And if you're listening to my voice, you're

listening to thousands of hertz, if not tens of thousands of hertz in my voice. And so

those waves are oscillating really, really quickly. Much quicker than these flashing

lights right here. Last thing in a wave is lambda. And what lambda simply is going to

be wave length. Wave length, well first let's look at this wave right here. In a transverse

wave it's going to have a crest which is going to be the top. It's going to have a trough

which is right here at the bottom. And then we're going to have the node which is right

in the middle. And so from crest to crest we call that one wave length. And what you'll

find is it's going to be the same distance from here to here, here to here. In other

words how long a wavelength is is going to be lambda. Or that's going to be the wavelength

of a wave. Now to really measure and play around with the waves, I would encourage you

to do this. This is a simulation. It's found at phet.colorado.edu. They do some wonderful

science animations. And this one is called waves on a string. So let me go find that

for a second. So here would be a wave that you can kind of play around with. So what

you can is we can grab this string of beads and I can move it up and down. And I can move

a wave from this side to that side. Now what you'll find, it's hard for me to do this very

well. Let me try that again. Is that the energy is being travelled from or is traveling from

point A to point B. But the beads aren't traveling. So it's just being transferred through that

medium. So this would be a typical wave. Let me just make a quick pulse like that. So what's

happening here? Well that wave is moving down and then it's just moving right out the door.

Let's kind of reduce the damping for just a second and see what happens. Alright. So

we even have more of a wave that's moving down. Now let's actually put a fixed end on

that. So now what I'm going to do is I'm going to actually send a wave down. And let's see

what happens to it. Ah. The wave is being reflected or it's bouncing back. And this

is a characteristic of waves as well. I'm going to put a damper on that for just a second.

So let's see what happens when I send a wave and then I send another wave. Well what happens

when they hit? That's kind of hard to see. Let's try that again. Let's say if I send

a wave. And then I send another wave, well, what happens when they hit? Well when they

hit they actually cancel each other out. It's hard to see. Let's see if we do it this way.

Let's make a loose end now. What happens if I have a loose end? We send that down. That's

cool. The wave actually comes back on the same side. So now let's send a wave and now

another wave down. What happens when they hit? Well what's happening is they're actually

taking the energy of both waves when they collide and then we're adding to that. So

we have that, that's called constructive interference. So now let's kind of oscillate our waves.

So let me keep the dampening like that. Let's go back to no end at the end. And now let's

set it to oscillate. So now what do we have in a wave? Well, in this wave we've got a

high amplitude. So amplitude is going to refer to how big the wave is. So let's reduce that.

So amplitude is how high it's moving up and down. Frequency is going to be how fast it

occurs. So right now the frequency is 50 hertz. What does that mean? We have one divided by,

what would that be. One divided by 0.02 seconds. And so we're getting 50 waves per second.

So that's going to be a high frequency. If I increase the frequency, so we're going to

have more waves per second or 50 waves per second. And I get it really cranking, now

we have 84 waves per second. Or 84 hertz. What happened to the wave length when I did

that though? So what happened to the wavelength? Well as I increased the frequency I decreased

the wavelength. So let's go back here. What happens when I decrease the frequency? So

now it's only 24 waves per second. Now the wavelength gets really really longer. So what

do we remember? Well v, so if we go back to our equation, v, which is speed, equals frequency,

which is how many hertz it is times lambda. So what does that mean? If we increase the

frequency, if we increase the frequency then the wavelength is going to go down. Now a

cool thing can happen if we actually add a fixed end to that. We start to get constructive

interference. And so what's happening now? Waves that are going down are meeting waves

that are coming back. And so if we increase the amplitude a little bit we can actually

get some really big waves. Now let's decrease the dampening. And now we've got waves that

are almost standing waves. Or dancing waves at this point. If we really reduce the dampening,

then this is going to get crazy out of control. And so that's waves on a string. And that's

phet. So let's go back to the keynote for just a second. And so there are a few more

properties that you should understand about waves. And the next thing is what happens

when they move from one medium to another. So there are essentially three things that

can happen. Four, but let's just talk about these three. It could be absorbed as well

by the material. But first is reflection. So what happens with a reflected wave? In

this case we're using a laser, which is coherent light. So we have light moving in this direction

which is a wave. It hits the surface that's reflected and we get a reflected wave. And

this reflect, angle of reflection is equal to the angle of incidence. So the waves are

simply bouncing off of it. So when you're looking at a mirror, those would be reflective

waves that are coming back to you. Now another thing that can happen when it moves from on

medium to another, in this case we're going from air it looks like to a lens or the this

could be filled with water. It's being refracted. What does that mean? It's being bent. And

so as the light moves in this direction, it's a wave, it hits this and it's actually slowing

down and that's bending or refracting the wave. Now you can also see here that some

of that is being reflected. But we certainly have a lot of refraction here or bending of

a wave. And then the last thing that occurs is something called diffraction. When you

move waves through a small opening, the waves will actually bend. And that's called diffraction.

So they're bending through this opening. Or they could be bending around the bend as well.

So let's say you're listening to music and you're right over here, that sound wave is

actually going to bend around so you can hear it. Now which waves are you going to hear

bend more easily? Well the high frequency waves, the high pitches will actually move

right through. But the low frequency will actually bend more quickly. And so that's

why when you hear a car coming by and they're listening to really loud music you'll hear

that boom, boom, boom, boom, boom. You hear that low frequency sound because it's diffracted

more readily to get out of the car. But the high pitches or the higher frequency noises,

they don't get diffracted as much. So you don't hear them. Last thing I want to show

you is you can solve simple problems. And so let's say this is a real world example.

Let's say we have a tsunami which is a giant wave in the ocean created by an earthquake.

Let's say it has a wavelength of 210,000 meters. So that's 210 kilometers between waves. That's

a huge wave length. And let's say it has a frequency of 0.00067 hertz. That would be

like 1 wave coming every 25 minutes. Now calculate the speed of the wave. Well how would you

do that? Well we remember our equation, v which is wave speed equals frequency times

wave length. We have to look at our units so we know frequency and it's in hertz and

so we're fine. We know wavelength and that's going to be in meters. And so we're fine.

So to figure out that the wave's speed, we simply multiply the frequency times the wavelength

to figure out the wave speed. And I did this earlier. When you take 210,000 times 0.00067

what you get is 140, if we do significant digits right. Because both of those have 2

significant digits. 140 meters per second. Now most of us don't understand what meters

per second are. So we can roughly take that times 2.2. And so a tsunami that has that

large of wavelength and that small of frequency is going to move at about 310 miles per hour.

And so these things move really really quickly. And that's why it's important that we know

and get to higher ground when we hear the tsunami warnings going. And so that's waves.

And I hope that's helpful.