Ch. 10,11, 13 Waves

Name some waves

• Water

• Sound

• Light

• Radio waves (TV)

• Microwaves (cell phones)

• X-rays

• Ultrasound

• Earthquakes! (seismic waves)

Definition

• Wave: repeating disturbance or movement that transfers energy through matter or space.

• How does it transfer energy?

• By making things MOVE

Mechanical Waves

• Mechanical Waves: travel through a medium - sound waves, water waves

• Medium: Solid, liquid or gas

Light doesn’t need a medium

Sound waves DO

Types of Mechanical Waves

• Transverse: side to side motion 90º to the direction of wave travel; ocean, seismic s-waves

• Compressional: back and forth in the direction of wave motion; sound waves, seismic p-waves

Other mechanical waves

• Water waves: combination transverse and compressional

• Seismic waves: earthquakes

s-waves = transverse

p-waves = compressional

Wave Properties - Transverse

• Wavelength: distance between 2 crests

Wavelength λ

Amplitude (at crest)

Trough

Wave Properties – CompressionalWavelength: distance between 2 compressions

Comparison

Compressional and Transverse

Transverse Wave

Compressional Wave

Water Wave

Watch the blue particle

Wave Characteristics

• Frequency- f: # of waves that pass a point per second.

• Unit = Hertz (Hz) = 1 / second

• Higher frequency = shorter wavelengths

• Period-T: Amount of time it takes for one complete wave to pass a point

• Unit = seconds

• T = 1/f

Amplitude and Energy

• Amplitude = height of crest or depth of trough OR

Amplitude = density of the compression • Greater amplitude = greater energy • Ever been knocked down by a BIG wave

at the beach?• Two waves with equal frequency and

wavelength – the larger amplitude wave has the greater energy.

Wave parts

• Transverse parts• Crest – top of the

wave (or a toothpaste)

• Trough bottom of the wave

• Wavelength- space between two crests or troughs

• Compression parts• Rarefaction- less

dense area• Compression- more

dense area• Wavelength- space

between compressions or refractions

Wave speed

• Different waves have different speeds

• Light is faster than sound waves

• Baseball stadium cheap seats: See the ball hit before you hear it

Wave Speed

• Sound moves faster in solids/liquids than air

• Light waves move more slowly in liquids

Wave Speed

• Calculating wave speed:

v = f∙λ

where

v = wave speed

λ = wavelength

f = frequency

Example

• λ = 4 m, f = 2 Hz, v = ?

v = f ∙ λ

= 4 m ∙ 2 Hz

= 8 m/s v

f λ

Example

• v = 14 m/s, f = 2 Hz, λ = ?

v = f ∙ λ

14 = 2 Hz x λ

14 / 2 = λ

λ = 7 m

v

f λ

Behavior of Waves

• Reflection: when a wave hits an object and bounces off

• Examples:

echos = sound waves

image in mirror = light waves

Law of Reflection

• Angle of incidence = Angle of reflection

Refraction

• The bending of a wave caused by a change in it’s speed as it moves from one medium to another

Air

Water

Example

• Standing in pool, legs look shorter

• Look at object at bottom of bathtub and try to grab it

Refraction

Diffraction

• Diffraction: occurs when an object causes a wave to change direction and bend around it

• Look closely at a shadow – transition area between light and dark

• Shine light through thin slit – see diffraction and interference

Diffraction

Interference

• when two or more waves overlap and combine to form a new wave

• Constructive: add to each other

• Destructive: subtract or cancel each other

Constructive Interference

Standing Waves

• wave pattern that forms when waves of equal wavelength and amplitude but opposite directions continuously interfere

• Nodes: zero amplitude

• Examples: musical instruments – strings, woodwinds, etc.

Standing Waves

Resonance

• when an object vibrates at it’s natural frequency.

• Example: ringing bell, tubular chimes, tuning fork

• Badly balanced tires on a car – resonance at certain speeds.

• http://www.archive.org/details/SF121