outline: what is a wave? what is a wave? properties of waves? properties of waves? check your...

Outline:Outline:

What is a wave?What is a wave? Properties of waves?Properties of waves? Check your understanding!Check your understanding!

What is a wave?What is a wave?

First thing comes to mind,

ripples spreading out

on water surface when you throw a rock in the

lake.

Ocean waves, sure those are waves!

The TV broadcast signals are

transmitted in forms of

electromagnetic waves

Sounds are waves too,

this is a picture of visualized

sound waves for some

noise.

Lights are waves, the natural sunlight contains 7 different colored lights. Each light wave appears

its own color is because they have

slight different properties.

All of them are waves!

Waves are everywhere,

these are just some examples.


A wave is a disturbance that travels from one location to another location.

Pay attention to What the person did to start the wave?This is what we meant by disturbance.

Sounds are in forms of waves

too. Can you explain how we make disturbances when we talk

(generate sound waves)?

Similarly, throwing a rock in the lake

is making a disturbance. Water ripples spreading out is the wave

traveling to further locations.


The traveling disturbance in a wave The traveling disturbance in a wave is carrying energy. is carrying energy.

Think about ocean waves. When the

wave hits shore, it has so much

energy that can sometimes

destroy trees and houses.

What happens to the boat when a wave

hits it? What happens after

the wave travels past it?

What happens to the boat that’s further

away?Carries energy to make the boats move

How exactly does a wave carry energy? Let’s make some waves with a rope

and find out!


Waves travel through medium.Waves travel through medium.

The string is the medium for the wave we just

made.Water molecules is the medium for ripples and

ocean waves.Air molecules is the medium for sound.

What happens if there isn’t a medium for the

wave to travel through? Say we make

a disturbance and there is no string. Or when we talk in outer

space.

Medium segments interact with each other to cause the next segment start moving, that’s how

energy is carried on.


Can you tell which ones are waves Can you tell which ones are waves and which ones aren’t? and which ones aren’t?

Point out the medium if it is a wave.Point out the medium if it is a wave.

If the man shakes his hand up and down, will he make a

wave? Jumping Rope? Is it carrying a

wave?


A mini waterfall, is it

a wave? What’s the difference between this and ripples?


When a gun is fired, does it

create waves?


Are wind waves?

What happens if

the dog blows on it?

What exactly is a wave?What exactly is a wave?

From the examples, we can see that a From the examples, we can see that a wave passes on energy, not matter. wave passes on energy, not matter.

Energy is passed along the wave, and Energy is passed along the wave, and make the medium vibrate.make the medium vibrate.

Because medium interact with each other, Because medium interact with each other, a vibrating medium segment causes the a vibrating medium segment causes the adjacent segment to vibrate, then the adjacent segment to vibrate, then the adjacent segment causes its adjacent adjacent segment causes its adjacent segment to vibrate, thus energy is passed segment to vibrate, thus energy is passed further.further.

Properties of wavesProperties of waves

Waves can look very different Waves can look very different depending on the source. depending on the source.

There are many more kinds of

waves. Here are just some examples.


Most waves in nature look like this. Most waves in nature look like this.

Recall the waves on the

string we made.

Do they look like this one?How does

water ripples fit into the

wave graph?

There are a lot of waves spreading out in

all the directions. If we take a cross-section, we will see how each wave look

like.

One Cross-section


Amplitude

Crest

Trough

How would the wave move?Recall the

wave on the string we

made.

The entire wave will move to the

right on the string, because you are making disturbance on

the left end.

Wavelength

Properties of wavesProperties of wavesIf we keep making the same disturbances, a

lot more of these wave segments will appear. And they will all look

the same.

Keep track of the red wave segment, this is the first wave generated.


If two waves have different If two waves have different properties, they are likely to appear properties, they are likely to appear different.different.

Remember we said lights are waves

too? Lights appear to be a certain color

is because they have different wavelength.


The brightness of a light depends on the amplitude.

Least bright

Ok bright

Brightest

Note, waves themselves do not have colors. It’s

the energy their carrying that excites our vision system so that our

brain see them in certain colorful ways.Dog’s brain can not do

that, so they can not see color.


For sound waves, the AMPLITUDE

determines how LOUD the sound is.

Wavelength determines the

PITCH of the sound.

Let’s make some noises and see for ourselves!Look at the first

row. One is louder than the other one. What

makes them different?

Look at the first column. One is

high pitch (note), the other is low

pitch. What makes them different?

Some Questions NowSome Questions Now

So far you have learned quite a bit So far you have learned quite a bit about waves. Now I will ask you some about waves. Now I will ask you some challenging questions to see if you challenging questions to see if you REALLY understood the concepts we REALLY understood the concepts we are dealing with here. Most of them are dealing with here. Most of them will require some independent will require some independent thinking or even guessing, so don’t thinking or even guessing, so don’t worry if you get them wrong. The worry if you get them wrong. The purpose is to get you to think! purpose is to get you to think!

Medium or No Medium?Medium or No Medium?

How can we see the sun from earth?How can we see the sun from earth?

Is there any medium to transmit the Is there any medium to transmit the light waves to earth?light waves to earth?

Wait a minute!How is it possible that these

waves can travel without medium???

Question: How can light Question: How can light waves travel WITHOUT waves travel WITHOUT

medium?medium?Hint:

Recall what medium did in the rope wave. Is there a way we can have some sort of way to

pass on energy without matter?Nature has a clever way of doing

it, guess what it is?The answer is ELECTROMAGNETIC WAVES !

When a light source gives out light , it generates electromagnetic waves to travel in all directions. Instead of a medium, these waves store energy in electric and magnetic fields, and pass on energy by shifting these fields.

Electric field

Magnetic field

Assorted Facts…Assorted Facts…

As you can see, the As you can see, the ELECTROMAGNETIC ELECTROMAGNETIC WAVESWAVES are very different from the other are very different from the other waves in the ways of traveling. waves in the ways of traveling.

We call the other waves We call the other waves MECHANICAL MECHANICAL WAVESWAVES because they need matter because they need matter (medium) to “mechanically” pass on the (medium) to “mechanically” pass on the energy.energy.

Ex: Ocean waves, ripples, sound waves, Ex: Ocean waves, ripples, sound waves, waves on a rope…waves on a rope…

Electromagnetic Waves?Electromagnetic Waves?

What waves are electromagnetic What waves are electromagnetic waves? (Hint: waves that do not waves? (Hint: waves that do not need medium to travel)need medium to travel)

Speed of a wave?Speed of a wave?

How do you calculate the speed of a How do you calculate the speed of a wave?wave?

“Hello!”

680 m

Tom and Marry are 680 meters

apart.

Tom says “Hello” to marry. It takes her 2 seconds to hear it.

What’s the speed of the sound wave?


Will it take more or less time for Will it take more or less time for Marry to hear Tom if they were Marry to hear Tom if they were talking in water?talking in water?

680 m

Hint: Think about the media the

waves travel in. Which one would carry on energy

faster?


For mechanical waves (such as For mechanical waves (such as sound waves), medium affects the sound waves), medium affects the speed of the wave. What about speed of the wave. What about electromagnetic waves?electromagnetic waves?

The speed of electromagnetic waves are not affected by the

medium they travel in. They don’t use it

anyways!

Unlike sound waves and waver waves have different speeds of their own,

ALL ELECTROMAGNETIC WAVES HAVE THE SAME SPEED!

---- 300,000,000 meters/second

Here are all the Here are all the electromagnetic electromagnetic waves you can think waves you can think of. They ALL have of. They ALL have the speed of the speed of 3x103x1088m/s.m/s.

Period?Period?

If a wave is moving to the right, how If a wave is moving to the right, how far does it have to move to make it far does it have to move to make it identical to the what it looks like identical to the what it looks like now?now?

Wavelength

The time it takes a wave to move a wavelength

long, is called PERIOD.We use T to refer to it.

Period, speed and Period, speed and wavelength?wavelength?

If a wave moves at a speed of v If a wave moves at a speed of v meters/second,meters/second,

and has a wavelength of and has a wavelength of λλ meters, meters, what is the period T?what is the period T?

λλ metersmeters

How much time to move this far?

Hint: time=distance/s

peed

Frequency and period?Frequency and period?

If I tell you a waves frequency is the If I tell you a waves frequency is the reciprocal of its period. What does reciprocal of its period. What does frequency represent physically?frequency represent physically?

frequency = 1/Tfrequency = 1/T

Hint: If a wave moves one

wavelength long every 3 seconds,

then for each second it moves 1/3 wavelength.