Waves, Sound and Light

Chapters 15 and 16

Standards:

SPS9. Students will investigate the properties of waves.SPS9a. Recognize that all waves transfer energy.SPS9b. Relate frequency and wavelength to the energy of different types of electromagnetic waves and mechanical waves SPS9c. Compare and contrast the characteristics ofelectromagnetic and mechanical (sound) waves.SPS9d. Investigate the phenomena of reflection, refraction,interference, and diffraction.SPS9e. Relate the speed of sound to different mediums.SPS9f. Explain the Doppler Effect in terms of everydayinteractions.

Waves

Waves• rhythmic disturbances that carry energy through

matter or space

Medium

• material through which a wave transfers energy

• solid, liquid, gas, or combination

• mechanical waves need a medium

• electromagnetic waves don’t need a medium (e.g. visible light, radio, tv)

Wave Characteristics

• transfer energy• the bigger the wave, the more energy carried• create an erosion force• most are caused by vibrating objects • tsunami: ocean wave caused by earthquakes• wave front: circles spreading out from a wave (each wave front carries same amount of

energy)

Waves

• Two Types:

Longitudinal Transverse

Anatomy of Waves

• crest: high points on transverse wave• trough: low points on transverse wave• compressions: crowded areas on longitudinal

wave• rarefactions: stretched-out areas on

longitudinal wave

Transverse Waves

Transverse Waves• medium moves

perpendicular to the direction of wave motion

ex. electromagnetic waves

Transverse Waves

• Wave Anatomy

crests

troughswavelength

wavelength

amplitude

amplitude

corresponds to the amount of energy carried by the wave

nodes

Longitudinal Waves

Longitudinal Waves (a.k.a. compressional)• medium moves in the same direction as wave motion ex. sound waves

Longitudinal Waves

• Wave Anatomy

rarefaction

compression

wavelength

wavelength

Amount of compression corresponds to amount of energy AMPLITUDE.

Measuring Waves

Frequency ( f )• # of waves passing a

point in 1 second• Hertz (Hz) unit

shorter wavelength higher frequency higher energy

1 second

Measuring Waves

Period• time it takes for 1 complete wave cycle

Measuring Waves

Amplitude:• greatest distance particles in wave move from

rest• larger amplitude greater energy

Measuring Waves- SpeedVelocity ( v )• speed of a wave as it moves forward• depends on wave type and medium

v = × f v: velocity (m/s): wavelength (m)f: frequency (Hz)

WORK:v = × f

v = (3.2 m)(0.60 Hz)

v = 1.92 m/s

Practice: Measuring Waves Find the velocity of a wave in a wave pool if its

wavelength is 3.2 m and its frequency is 0.60 Hz.

GIVEN:

v = ? = 3.2 mf = 0.60 Hz

v

f

WORK: f = v ÷

f = (5000 m/s) ÷ (417 m)

f = 12 Hz

Practice: Measuring Waves

An earthquake produces a wave that has a wavelength of 417 m and travels at 5000 m/s. What is its frequency?

GIVEN:

= 417 m

v = 5000 m/s

f = ?

v

f

Measuring Waves: Speed-Period

• v = λ T λ – wavelength T -- period V T

λ

Measuring Waves: Frequency-Period

Frequency-Period• period: time it takes for a wave to pass a

certain (T) related to... • frequency: number of wavelengths that pass a

given (f)• f = 1 period

f

1

T

Wave Speed Facts

• depends on medium: Fastest- solid > liquid > gas –slowest• speed of light (c) = 3.00 x 108 m/s (finite speed)• visible light is detected by eye• Full light range = electromagnetic spectrum• speed of sound in air = 340m/s

Visible Light

The Doppler Effect

• Doppler Effect: change in frequency of a sound wave when the source or observer is moving

• pitch (how high or low): determined by frequency at which sound strikes eardrum

FYI

• Doppler radar uses radio wave frequency shifts to track storms since radio waves reflect off of rain, snow and hail

Wave Interactions

• Reflection: bouncing back of wave when it meets a boundary

• Refraction: bending of waves when they pass from one medium to another

• Diffraction: bending of waves when they pass around an edge

RefractionReflection Diffraction

InterferenceCombination of two or more waves thatcombine into a single wave:• Constructive- increases amplitude

• Destructive- decreases amplitude (cancels each other out)

Light Interference

• constructive and destructive waves create different frequencies (colors)

ex. rainbow seen in oil on water, iridescent colors on peacock feather

Sound Interference

• When wave compressions from 2 sources arrive at ear at same time = louder sound (constructive interference)

• When wave compression and rarefaction from 2 sources arrive at ear at same time = beat

(destructive interference)

Standing Waves

• Results from interference between wave and its reflection

• Causes medium to vibrate in a stationary pattern (loop or series of loops)

• Nodes: crest of wave meets its reflected trough (complete destructive interference)

• Antinodes: crest of wave lines up with reflected crest; points of maximum vibration;

(complete constructive interference)

Standing Waves

Properties of Sound

• longitudinal waves• require medium • spread in air in all directions from source• travel slower in gas; faster in most solids (foam, rubber damper vibrations) • travel faster at hot temperatures (greater

collision of molecules)

Speed of Sound

Pitch

• Pitch: wave frequency • ↑ f = ↑ pitch • Range of pitch for humans: 20hz – 20,000hz• Infrasound: sound below human hearing• Ultrasound: sound above human hearing

Ranges of Hearing for Mammals

Loudness

• Loudness: determined by intensity (amplitude and distance from sound source)

* measured in decibels, dB * threshold of human hearing- 0 dB * threshold of pain- 120 dB

Musical Instruments

• Produce sound through vibrations of string, air columns, membranes

• Rely on standing waves• Use resonance to amplify sound• Resonance: when two objects naturally vibrate

at same frequency (depends on size, shape, mass and materials)

(electric guitars don’t resonate well so they require separate amplifiers)

Hearing and the Ear

Senses vibrations, amplifies them, transmitsthem to the brain:• Outer ear: Pinna collects sound waves, sends to

ear canal, causes tympanum to vibrate• Middle ear: vibrations pass to hammer, anvil,

stirrup (small bones act as levers to increase vibrations)

• Inner ear: vibrations in cochlea are converted into electrical signals to brain

The Ear

Ultrasound

• High f of ultrasound can travel through most material

• Used to measure distance

• Reflected waves create image

• Sonagram: used in medicine to view internal organs

Ultrasound-Sonar

• Sound navigation and ranging

• Uses reflected sound waves for measurement of distances

• Used by marine mammals

Sound Project

With a partner: Create a musical instrument from scratch that can produce a recognizable tune (ex. Mary Had a Little Lamb, Row, Row, Your Boat, Beethoven’s Fifth Symphony)

• Suggested Materials: 5-8 bottles water• Time: 2 class periods

The Nature of Light

Has dual nature:1. Thomas Young’s experiment showed light moves in

electromagnetic waves *explains how light waves interfere with each other2. Light can also be modeled as a stream of particles * photons: bundles of high energy light units

Properties of Light

Electromagnetic Radiation• transverse waves produced by motion of

electrically charged particles• does not require a medium

Speed of Light

• depends on medium• ≈ 3.0 x 10⁸ m/s in a

vacuum (nothing known is faster)

• travels slower outside a vacuum

(1.24 x 10⁸ m/s through a diamond)

Brightness

Intensity: measure ofbrightness• decreases with distance

from light source due to decrease in photons passing through an area

Electromagnetic Radiation

• made up of electric and magnetic particles• consists of waves of all possible energies,

frequencies and wavelenghts• each part of spectrum has unique qualities• used in technologies

Types of EM Radiation

Radio waves:• longest wavelengths• lowest energy EM radiation• Include TV and radio signals- AM (amplitude

modification), FM (frequency modification) • Radar: radio detection and ranging

Types of EM Radiation

Microwaves:• carry telecommunication signals long

distances• penetrate food, vibrate water & fat molecules

to produce thermal energy

Types of EM Radiation

Infrared Radiation (IR)• slightly lower energy than

visible light• can raise the thermal energy

of objects • felt as warmth• thermogram - image made by

detecting IR radiation

Types of EM Radiation

Ultraviolet Radiation (UV)• slightly higher energy than visible light• Types: UVA - tanning, wrinkles UVB - sunburn, cancer UVC - most harmful,sterilization• absorbed in ozone layer

Types of EM Radiation

X rays• higher energy than UV• can penetrate soft tissue,

but not bones

Types of EM Radiation

Gamma rays• highest energy

EM radiation• emitted by

radioactive atoms• used to kill cancerous

cells (kills healthy cells too) Radiation treatment using

radioactive cobalt-60.

Types of EM Radiation

• Visible Light– small part of the

spectrum we can see

– ROY G. BIV - colors in order of increasing energy R O Y G. B I V

red orange yellow green blue indigo violet