# Chapter 1.

## Presentation on theme: "Chapter 1."— Presentation transcript:

Chapter 1

Imagine…

The material that a wave travels through is called a medium.
What’s the medium in the story?

Waves that require a medium to travel through, like the one we mentioned in the story, are called mechanical waves.

But not all waves require a medium…
Light from the sun can carry energy through empty space. Waves that travel without a medium are called electromagnetic waves. More about that later…

Let’s practice being particles in motion. I’ll be the energy source (the wind in this case) that causes you to vibrate and you’ll be the water.

You just formed a transverse wave!
As a transverse wave moves, the particles of the medium move across, or at a right angle to, the direction of the wave. Let’s take another look.

Transverse Wave

Parts of a Transverse Wave
Crest- the highest part of the wave Trough- the lowest part of the wave

Another type of mechanical wave…
Longitudinal Wave Let me demonstrate… These waves move the medium parallel to the direction in which the waves travel. In this example, the coils in the Slinky move back and forth parallel to the motion of the wave.

Parts of a Longitudinal Wave
Compression- parts where the particles of the medium are close together Rarefaction- parts where the particles of the medium are spread out, or rarified Sound is a longitudinal wave. In air, sound waves cause air particles to move back and forth.

Part 2

Think back to yesterday when you formed a transverse wave with the class…
Remember when we “sent the wave back” across the row? This is called reflection. Reflection is when a wave or object hits a surface through which it cannot pass and therefore it bounces back.

What are some types of reflection that you can think of?
Echo (reflection of sound) Mirror (reflection of light) Ball bouncing (reflection of an object)

Refraction (NOT Rarefaction or Reflection!)
When a wave enters a new medium at an angle, one side of the wave changes speed before the other side, causing the wave to bend, or refract. The bending of waves due to a change in speed is called refraction. This only happens when the wave enters the new medium at an angle.

What are some examples of refraction that you can think of?

Another type of wave bending…
When a wave moves around a barrier or through an opening in a barrier, it bends and spreads out—this is called diffraction. Let’s look at a few examples of diffraction…

Two waves can overlap when they meet.
Interference is the interaction between waves as they meet. There are two types of interference: Constructive Destructive

Constructive Interference
A picture (or a video) is worth a thousand words... Constructive interference is when waves combine to make a larger wave.

Destructive Interference
This is when 2 waves combine to make a smaller wave. Destructive interference occurs when the crest of one wave overlaps the trough of another wave. Basically, the waves are cancelling out each other’s energy.

Standing Waves http://youtu.be/NpEevfOU4Z8 http://youtu.be/uFwEGrxNp5Q
If an incoming wave and a reflected wave have just the right frequency, they produce a combined wave that appears to be standing still even though it is really two waves interfering as they pass through each other.

Nodes and Antinodes Node- a point of zero amplitude on a standing wave
Antinode- a point of maximum amplitude on a standing wave

Part 3

Properties of Waves amplitude Wavelength Frequency speed

Amplitude- the greatest distance that the particles of the medium move from their rest positions

In a transverse wave… The amplitude is a measure of the greatest distance up or down from the rest position. The larger the distance is from rest position to crest or from rest position to trough, the higher the energy of the wave.

In a longitudinal wave…
The amplitude is a measure of how compressed or rarefied the medium becomes. A high-energy wave causes more compression and rarefaction than a low energy wave. When the compressions are dense, it means that the wave’s amplitude is large.

Wavelength- the distance between two corresponding parts of a wave
crest to crest or trough to trough in a transverse wave compression to compression in a longitudinal wave

Frequency-the number of complete waves that pass a given point in a certain amount of time

The faster the wave movement, the higher the frequency and the slower the wave movement, the lower the frequency Frequency is measured in a unit called Hertz (Hz), named after the guy who discovered radio waves

Speed Different waves travel at different speeds.
For instance, which happens first during a thunderstorm—you see the lightning or you hear the thunder? Let’s take a look.

Why do you see the light waves before you hear the sound waves?
Because they travel at different speeds! Light travels about 1,000,000 times faster than sound! The speed of a wave is how far the wave travels in a given length of time.

Speed=wavelength x frequency
Time for some wave math! The formula used to relate speed, wavelength, and frequency of a wave is: Speed=wavelength x frequency Let’s do some practice…

What is the frequency? The speed of a wave on a rope is 50 cm/s and its wavelength is 10 cm. The speed of a sound wave through the air is 340 m/s and the wavelength is 18 ft/s. *Hint: There are about 3 feet in a meter.

What is the speed? A wave has a wavelength of 2 mm and a frequency of 3 Hz. A wave in a spring has a wavelength of 0.1m and a frequency of 20 Hz.

What is the wavelength? A sound wave has a frequency of 660 Hz and its speed is 330 m/s. The speed of a wave on a guitar string is 142 m/s and the frequency is 110 Hz.

Part 3

Resonance An increase in the amplitude of a vibration that occurs when external vibrations match an object’s natural frequency

You made it! The End 