INTERACTIONS INTERACTIONS OF WAVESOF WAVES Interference Interference RefractionRefraction DiffractionDiffraction Doppler EffectDoppler Effect PolarizationPolarization

ReflectionReflection

When an object or wave hits a surface When an object or wave hits a surface through which it cannot, pass it bounces through which it cannot, pass it bounces back. This is called reflection.back. This is called reflection.

Law of ReflectionLaw of Reflection

This law states the angle of reflection is This law states the angle of reflection is congruent to the angle of incidence.congruent to the angle of incidence.

ReflectionReflection

Waves bounce back at a boundary.Waves bounce back at a boundary.

The reflected wave is exactly like the The reflected wave is exactly like the incident wave.incident wave.

At a fixed boundary , the wave will reflect At a fixed boundary , the wave will reflect but it will turn upside down. but it will turn upside down.

Fixed boundary Fixed boundary

INTERFERENCEINTERFERENCEwhen 2 or more waves meet while traveling in the same medium (overlapping waves)

the NET displacement (amplitude) is the sum of the amplitude of the waves

Constructive or destructive interference

Creates standing waves & beats

CONSTRUCTIVE INTERFERENCECONSTRUCTIVE INTERFERENCE

larger displacementlarger displacement (larger amplitude)(larger amplitude)

crest overlaps crest crest overlaps crest

or or tough overlaps troughtough overlaps trough

DESTRUCTIVE INTERFERENCEDESTRUCTIVE INTERFERENCE

smaller displacementsmaller displacement (smaller amplitude)(smaller amplitude)

crest cancels trough crest cancels trough

crest overlaps trough crest overlaps trough

displacement/ displacement/ amplitude is reducedamplitude is reduced

In music In music interference creates “beats” interference creates “beats” (changes in loudness & softness)(changes in loudness & softness)

BEATS: BEATS: INTERFERENCEINTERFERENCE

Wave 1 (red) and Wave 1 (red) and

Wave 2 (blue) Wave 2 (blue)

combine to form beat pattern (in green)combine to form beat pattern (in green)

High amplitude = High amplitude = LOUDLOUD

Low amplitude = Low amplitude = soft

Overlapping waves of different f Overlapping waves of different f (frequencies)(frequencies)

STANDING WAVES:STANDING WAVES: INTERFERENCE INTERFERENCE

Points that have no displacement Points that have no displacement NODES NODES Large, moving displacements Large, moving displacements ANTINODES ANTINODES

(max. amplitudes)(max. amplitudes) String instrumentsString instruments

when two overlapping waves traveling in opposite direction have the same frequency

CONSTRUCTIVE INTERFERENCECONSTRUCTIVE INTERFERENCE

Resonance

When an object vibration at a particular frequency

Seen in musical instruments, especially strings

Sympathetic resonanceWhen one vibrating object induces a vibration in another object

Soldiers do not walk in formation across bridges

Tacoma Narrows Bridge

REFRACTIONREFRACTIONwhen a wave changes direction (& speed) as it passes from one medium into another

Refraction depends upon density of medium

Greater densities mean greater refraction (i.e. more bending)

REFRACTIONREFRACTION

What happens to the ray as it enters the water with less of an angle?

REFRACTION IN NATUREREFRACTION IN NATURE

Wave speed depends on the medium

Click on picture for internet animation

DIFFRACTIONDIFFRACTION

familiar examples: sound waves (speakers in a room) and water waves (breaker opening in harbors)

the amount of diffraction the amount of diffraction increases with increases with increasing increasing λλ

when a wave bends around an edge or spreads through an opening

DIFFRACTIONDIFFRACTION

Being able to hear around corners

Happens with light waves too (affects photography)

How is DIFFRACTION different How is DIFFRACTION different from REFRACTION?from REFRACTION?

Diffraction = wave bends around an edgeDiffraction = wave bends around an edge

Refraction = wave bends because it changes Refraction = wave bends because it changes from 1 medium to another (and, therefore, from 1 medium to another (and, therefore, changes speed).changes speed).

http://www.acoustics.salford.ac.uk/feschools/http://www.acoustics.salford.ac.uk/feschools/waves/diffract.htmwaves/diffract.htm

A bug moving to the right and producing disturbances

A• •B

A stationary bug producing disturbances in water

A• • B

apparent increase in ƒ for observers as the source approaches/ moves closer to observer

apparent decrease in ƒ for observers as the source recedes / moves away from observer

DOPPLER EFFECTDOPPLER EFFECTthe shift in frequency of wave when a wave source and an observer are moving relative to each other

Notice how waves near B are “compressed” → ƒ higher

Waves “leaving” A are “expanded” → ƒ lower

DOPPLER EFFECT: SOUNDDOPPLER EFFECT: SOUND

• Longer λ for observers as train leaves

• Shorter λ for observers as train approaches

DOPPLER EFFECT: LIGHTDOPPLER EFFECT: LIGHT

• Longer λ for observers as train leaves = light shifts RED

• Shorter λ for observers as train gets closer = Light shifts BLUE

POLARIZATIONPOLARIZATIONwhen the displacement of a transverse wave is in a specific orientation or direction

familiar example: polarized sunglasses

polarized materials only allow waves that correspond to a special direction to pass through

PRACTICEPRACTICEWord bank:

ReflectionReflection

RefractionRefraction

DiffractionDiffraction

InterferenceInterference

PolarizationPolarization

Doppler EffectDoppler Effect

PRACTICEPRACTICEWord bank:

ReflectionReflection

RefractionRefraction

DiffractionDiffraction

InterferenceInterference

PolarizationPolarization

Doppler EffectDoppler Effect

INTERFERENCE INTERFERENCE RESOURCESRESOURCES

http://www.kettering.edu/~drussell/Demos/http://www.kettering.edu/~drussell/Demos/superposition/superposition.html for applets. superposition/superposition.html for applets.

http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=35 http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=35 for applet.for applet.

To change frequencies, etc., enter new value, and RETURN.To change frequencies, etc., enter new value, and RETURN. To pause, left click.To pause, left click.

For Harmonics & Beats applets link to The Soundry For Harmonics & Beats applets link to The Soundry Sound Lab (left sidebar)Sound Lab (left sidebar)http://library.thinkquest.org/19537/http://library.thinkquest.org/19537/