Waves!

I. A little vocab…• WAVE:

– A periodic disturbance in a solid, liquid or gas

• MEDIUM:– Matter or space through which waves

travel

II. Types of waves1. Waves can be classified

by what they move through

a. Mechanical- Requires a medium

b. Electromagnetic-Does not require a medium

– Ex- light waves, radio waves, sound waves…

2. Waves can also be classified by how they move

a. Transverse waves – travel perpendicular to the direction of the wave

b. Longitudinal waves (aka compression waves )

travel parallel to the direction of the wave

Surface Waves: Have BOTH

• Occur at the border between two mediums…

• Water waves: Particles at the surface move parallel and perpendicular (in circles)

• The ball up and down and side to side

Draw your pictures

Waves Carry Energy• Simple Harmonic Motion- When you pull on a single

spring, its mass continues to bounce up and down.

• When pulled down, spring gains PE (elastic) As it moves back up when released, PE is transferred to KE

• Mass goes past the original starting position• Back at the top, all energy is transferred to PE (gravitational &

elastic PE)• When it moves down again, energy transfers back to KE and it

keeps going

Damped Harmonic Motion- a vibration that fades out as energy is transferred from one object to another • When pulled down, the

spring gains PE• Again, it will be

converted to KE when released

• BUT instead of that KE staying in that spring, it is transferred to the next spring in the line.

Anatomy of a Transverse Wave

Crest:Trough:

Anatomy of a Longitudinal Wave

Compression:Rarefaction:

How do we describe waves?

• Wavelength• Amplitude• Period• Frequency

Wavelength ()

– The distance between two identical parts of a wave

Draw 2 waves – one with a long wavelength and one with a short wavelength!

Amplitude- The greatest distance that particles are displaced from their

original resting positions – Distance from the middle of the wave to the crest

• (or half the distance between the crest and the trough)

– Larger waves have larger amplitudes and carry more energy

– Sound Energy:

Low amplitude Medium Amplitude HIGH Amplitude

Draw 2 waves – one with an amplitude of 5 mm and one with an amplitude of 2 mm

Period

• The time required for 1 full wavelength of a wave to pass a certain point

– Or the time required to complete one vibration of a particle in a medium

• Represented by “t” (seconds)

Frequency

The number of cycles/wavelengths/vibrations per unit of time

– Represented by “f” ; f= 1/t– Measured in Hertz (named after Heinrich Hertz – 1888

– 1st demonstrated the presence of electromagnetic waves)

– Equal to vibrations per second (1Hz = 1 vibration/1 second)

• You can hear 20 Hz to 20,000 Hz

• More vibrations = shorter amount of time for each

1. Draw a wave with a period of .5 seconds and a frequency of 2 Hz

2. Draw 2 waves – one with a high frequency and one with a low frequency:

Frequency of Light Spectrum (example)

• Wide range of frequencies and wavelengths• Your eyes can detect 4.3 x 1014 Hz to 7.5 x 1014 Hz (visible

light range)• Electromagnetic waves exist at frequencies we can’t see

Remember that each wave carries energy…• Higher frequency = more energy and

shorter wavelength…The more wavelengths per unit time, the more energy.

Ex- sound waves…• Sound waves – travel as spherical

waves from a speaker• Wave front – each circular wave• As they travel out, the wave front

gets bigger – each front has the same amount of energy. As the wave front gets bigger, the energy is spread out over a bigger area (so that is why these waves are damaging close-up –close to speaker or close to light)

Wave speed – how fast a wave moves

• Speed = wavelength / period• So…• Wave speed = frequency x

wavelength (because the period is the inverse of the frequency

Wave speed depends on medium Sound waves can travel in air

However, they travel even faster in solids than in air or water

Example: hitting a metal railing and listening (you will hear two bangs)

Why?

The first sound comes through the steel rail itself and reaches you shortly before the second sound, which travels through the air.

Wave speed depends on medium

In a gas, particles are farther apart so it takes longer for them to find each other and bump into each other - sending the wave

• In liquids, particles are closer together so waves travel faster

• In solids, particles are very close together so waves travel the fastest

Example: Earthquakes(you don’t need to know

this…yet)Produce seismic waves

P waves (primary) – travel faster, arrive first, longitudinal waves (shake everything side-to-side)

S waves (secondary) – slower, carry more energy, transverse waves (shake everything up and down – more damage)

10. Doppler Effect

– First – pitch is determined by frequency– Sound waves travel out from an object in wave fronts

and hit your eardrum (when they hit your eardrum, they have a frequency equal to the number of wave fronts that hit your ear

– Ambulance example: • When the ambulance moves toward you, the sound

waves are compressed because the ambulance moves a short distance – so the waves hit your eardrum at a higher frequency – higher pitch

• Then, when the ambulance moves away from you, the distance between the wave fronts is more than before, so the frequency is lower – pitch is lower

– Doppler effect can also happen in light and other types of waves

• http://physics.bu.edu/~duffy/semester1/c20_wave_speed.html

• wave speed different medium• http://www.teachersdomain.org/resource/ess05.sc

i.ess.watcyc.wavemotion/• surface wave• http://www.acoustics.salford.ac.uk/feschools/wave

s/wavetypes.htm• transverse right angles• http://www.acoustics.salford.ac.uk/feschools/wave

s/reflect.htm#reflect• reflection