Waves 1Chapter 25

Vocabulary

Wave Vibratory disturbance that propagates (moves)

through a medium

Pulse Single disturbance

Medium Material through which a wave propagates

Waves

Waves transfer energy from one place to another, not mass

Wave Types

Two main types

Transverse Motion of the disturbance is perpendicular to the

direction of the wave propagation

Longitudinal Motion of the disturbance is parallel to the

direction of the wave propagation

Transverse Waves

Motion of the disturbance is perpendicular to the direction of the wave propagation

Example: Light

TRANSVERSE WAVES

Longitudinal Waves

Motion of the disturbance is parallel to the direction of the wave propagation

Example: Sound

LONGITUDINAL WAVES

Surface Waves

Combination of transverse and longitudinal waves

Example: Water

Water Waves (surface)

Wave Characteristics

Amplitude, A (m) Displacement away from equilibrium point

Wavelength, λ (m) Length of 1 wave cycle

Period, T (s) Amount of time for 1 wave cycle

Wave Characteristics (cont)

A

λ (m)

T (s)

Crest

Trough

Wave Characteristics (cont)

Frequency, f (Hz or s-1) Number of cycles per second Inverse of period

Speed, v (m/s) How fast wave is traveling Related to frequency (period) and wavelength

Equations

f = frequency (Hz) T = period (s) v = speed (m/s) λ = wavelength (m)

fT

1

v f vT

Light

Light is also called electromagnetic radiation Light is a combination of fluctuating electric

fields and magnetic fields that are perpendicular to each other

Electromagnetic Spectrum

Electromagnetic Spectrum

R Radiowave M Microwave I Infrared V Visible U Ultraviolet X X-Rays G Gamma C Cosmic

WavelengthDecreases

FrequencyIncreases

EnergyIncreases

Light (cont)

Transverse Wave Travels through vacuum

Color is based on frequency Green Light = 5.6 x 1014 Hz

Speed of light in a vacuum (air also) c = 3 x 108 m/s

Sound

Longitudinal Wave Needs a material (medium) to move

Pitch is based on frequency Concert A = 440 Hz

Speed of Sound in air is dependent on Temp v = 331 m/s at STP

Wave Speed

Waves must follow the kinematic equation

The speed of waves depends upon the material that the wave travels through

t

dv

Wave Speed

Sound can not travel in a vacuum, light can

Light travels fastest in a vacuum, slower in all other materials

Sound travels faster in more dense materials

Phase Difference

Two points are considered “in phase” when they are at the same point in a wave cycle

The amount of “in or out of phase” is measured in degrees

Phase Difference Examples

What point is in phase with A? B and D are how far out of phase? Name two other points in phase with each

other.

Wave Motion

Waves propagate in all directions without barriers

Wave Fronts

Line that represents waves that are all in phase, usually crests

Principle of Superposition

When two waves meet, they combine together briefly, then go their separate ways

Crest + crest = bigger amplitude Trough + trough = bigger amplitude Crest + trough = lower amplitude

Interference

Constructive Interference When 2 waves interfere with resultant wave

having larger amplitude

Destructive Interference When 2 waves interfere with resultant wave

having smaller amplitude

Simulation Examples

http://www.surendranath.org/Applets/Waves/TWave02/TW02.html

Interference Example

Two point sources (green dots) What do the red dots represent? What do the blue dots represent?

Sound Beats

Interference produced when two sounds interact

Frequency of beats is equal to difference of frequencies of two sounds

Concept used to tune pianos

Demo

Standing Waves

Occurs when two waves traveling in opposite directions in the same medium, with the same amplitude and same frequency

Resultant wave appears to be standing still

Demo

Nodes and Antinodes

Nodes Points of maximum destructive interference

Antinodes Points of maximum constructive interference

Nodes and Antinodes

Nodes and Antinodes

Doppler Effect

Change in frequency due to moving wave source or observer

Example

Example

Doppler Effect

When distance between source and observer is decreasing, frequency increases Blue Shift

When distance between source and observer is increasing, frequency decreases Red Shift

Sonic Boom

When moving object exceed the speed of sound, air builds up into a shock wave

Sonic Boom

Video

YouTube Video

How does this work?

Resonance

Natural Frequency Particular frequency that every elastic body will

vibrate at if disturbed

Resonance Vibration of a body at its natural frequency

because of the action of a vibrating source of the same frequency

Real Life

Microwaves produce waves that have the same frequency as the vibrational frequency of water molecules

UV rays have the same frequency as certain chemicals in human skin, causing sun burns

Google – Tacoma Narrows Bridge

Harmonics

Fundamental Frequency(1st Harmonic) Lowest frequency possible

2nd Harmonic 2x frequency of 1st Harmonic (Octave higher)

Closed Pipe Harmonics (Lab)

1st Harmonic L = 1/4 = 4L

3rd Harmonic L = ¾ = 4/3L

5th Harmonic L = 1 1/4 = 4/5L

Open Pipe Harmonics

1st Harmonic L = ½ =2L

2nd Harmonic L =

3rd Harmonic L = 1 ½ =2/3L