Waves

Objective: I will understand the difference between mechanical

waves and electromagnetic waves.

What is a wave?

A wave is a moving disturbance that transfers energy through matter or

space.

What causes waves?• Waves are created when a source of energy

causes a medium to vibrate.• A vibration is a repeated back and forth or up

and down motion.

Examples of waves

Water waves light waves

Sound waves microwaves

Radio waves X ray waves

Ultraviolet waves seismic waves

All of these waves can be put into 2 categories:

1. Waves that do not require a medium

2. Waves that require a medium

So what’s a medium?

A medium is something through which a wave travels. (solid, liquid, or gas!)• In a water wave, the medium is the water.• In a sound wave, the medium is the air.• In an earthquake, the medium is the ground.

Does light require a medium?

Well, where does light come from?

If it can travel through NOTHINGNESS, then it doesn’t need a medium

How do mediums affect movement?

• Waves move through different mediums at different rates:

• Solid: a wave moves through a solid medium the fastest

• Liquid: a wave moves through a liquid medium slower than a solid, but faster than a gas

• Gas: a wave moves through a gas medium the slowest

Mediums

• The medium can affect the speed of the wave travelling through it:

• An increase in the temperature of a medium increases the speed.

• An increase in the density of a medium increases the speed.

Why do waves move through different mediums at different rates?

• Particles in a solid are closer together so they can move to each other quicker.

• Particles in a gas are farther apart so the energy takes longer to move from particle to particle.

Mediums

As a wave moves the particles of matter do not move, only the energy is transferred.

Back to our 2 groups1. Electromagnetic waves can transfer energy

through a medium OR through empty space (a vacuum)– Ex. Radio waves, light waves, infrared waves, ultraviolet

waves, x-rays, gamma rays

2. Mechanical Waves are waves that require a medium in order to transfer energy.– Ex. Sound waves, water waves, seismic waves

Review

1. What is a medium?2. Do electromagnetic waves require a

medium?3. Give an example of an electromagnetic wave.4. In which medium do waves move the fastest?5. Do mechanical waves require a medium?6. Give an example of a mechanical wave.

Mechanical wave Electromagnetic wave

Transverse and Longitudinal WavesObjective: I will be able to

differentiate between transverse and longitudinal

waves.

2 different ways to create a wave:

Now lets see these two wave types in action

http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

Let’s look back at the wavesNow, when you watch the waves, notice how the particles of the medium are moving in different ways.

Note:• Which direction is the wave moving in?• Which direction are the particles moving in?• Are these two directions the same?http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

Waves• Types of waves: Waves are classified according to how they move.

Longitudinal Waves(Compressional waves)

In a longitudinal wave, the particles of the medium vibrate in the same direction as (or parallel to) the direction that the wave is travelling Examples: slinky, sound

Longitudinal waves (where particles move parallel to the direction of the wave) are something you experience every day in the form of SOUND!

Longitudinal/Compressional Wave

Parts of a Longitudinal/Compression wave:•Compression – space in a medium in which molecules are close together.•Rarefaction- space in a medium where there are fewer or less molecules. The molecules are more spread out.Think of a slinky!!!

Longitudinal/Compressional Wave

Transverse Waves

In a tranverse wave, the particles of the medium vibrate in the opposite direction of (or perpendicular to) the direction that the wave is travelling

Transverse waves (where the particles move perpendicular to the direction of the wave) are also all around you in the form of LIGHT!

Transverse WaveCrest

Trough Equilibrium/Resting Point

Wavelength

Wavelength Amplitude

Review

1. In a ___________ wave the particles of the medium vibrate PERPENDICULAR to the direction the wave is travelling.

2. In a ___________ wave the particles of the medium vibrate Parallel to the direction the wave is travelling.

3. Sound is an example of a ________ wave.4. Light is an example of a ________ wave.

Transverse Wave Longitudinal Wave

Wave Parts

Objective: I will be able to identify the main parts of a wave.

Basic Properties of Waves

AmplitudeWavelengthFrequencySpeed

Wavelength

• A wave travels a certain distance before it starts to repeat. The distance between two corresponding parts of a wave is its wavelength.

• Transverse measure from crest to crest or trough to trough.

• Longitudinal measure from one compression to the next.

Wavelength

The distance between one point on a wave and the exact same point on the next wave

Wavelengths

Transverse Wavelength

Longitudinal Wavelength

AmplitudeThe maximum extent of vibration or the distance from the resting position of the medium to the crest or trough.

Amplitude• The amplitude of a transverse wave is the

maximum distance the medium moves up or down (vibrates) from its rest position.

• The farther the medium moves as it vibrates the larger the amplitude of the resulting waves. The greater the amplitude the greater the amount of energy

Amplitude of a Longitudinal Wave

Frequency

The number of complete waves that pass a given point in a certain amount of timeAKA: the number of vibrations.Measured in Hertz

Crest and trough

The highest/lowest point of a transverse wave

Compression and rarefaction

The squished up or spread out parts of a longitudinal wave

Review

1. Draw a transverse wave and label – Wavelength– Amplitude– Crest – Trough

Review, continued

2. Draw a longitudinal wave and label– Wavelength– Compression– rarefaction

Practice

Practice

Measuring Waves• The speed, wavelength, and frequency of a wave

are related to each other by a mathematical formula.

• Speed = wavelength x frequency• Frequency = speed/wavelength• Wavelength = speed/frequency• Waves in different mediums travel at different

speeds. However, in a given medium and under the same conditions the speed of the wave is constant.

Measuring Waves•Read pages 499-501 in the purple Physical Science Book and do the 3 practice problems in your notebook.

Answers•1. 1.2 m X 4.5 Hz = 5.4m/s•2. 10m X 2.0 Hz = 20 m/s•3. 25.6 m/s / .20 m = 128

hertz

Ways Waves InteractReflectionRefractionDiffractionInterference

ConstructiveDestructive

Standing Waves

• When an object or wave hits a surface through which it cannot pass, it bounces back.

• Angle of incidence• Angle of reflection

Reflection

Examples of reflection

Refraction

• Though all waves change speed when they enter a new medium. Bending occurs when one side of the wave enters the new medium before the other side

REFRACTION•The bending of waves due to a change in speed is called refraction.

Diffraction

Interference

Constructive interference occurs whenever two waves combine to make a wave with a larger amplitude.

Destructive interference when the amplitudes of two waves combine producing a smaller amplitude.

Resonance

• Most objects have a natural frequency of vibration. Resonance occurs when vibrations traveling through an object match the object’s natural frequency.

• An object that is vibrating at its natural frequency absorbs energy from the objects that vibrate at the same frequency. Occurs in music.