Waves, Sound and Light Wave Properties. What is a wave? Disturbance or vibration that transmits energy but not matter Examples… Sound, light, radiowaves,

Waves, Sound and Light

Wave Properties

What is a wave?

• Disturbance or vibration that transmits energy but not matter

• Examples…• Sound, light, radiowaves, earthquakes

Types of Waves

• Mechanical Waves- Use a medium• Transverse waves- particles move

perpendicular to wave motion• Longitudinal Waves- particles move

parallel to wave motion

Parts Of A Wave

• Crest-top of the wave• Trough- Bottom of the wave• Amplitude (A)-height from resting position• Wavelength (λ)- distance travelled by a

single wave

Frequency and Period

• Period (T)- time for one complete cycle or wavelength (in s)

• Frequency (f)- number of waves per second (Hertz, Hz)• Hertz=1/s or s-1

• Frequency and period are reciprocals of themselves

• f= 1/T • T= 1/f• They are inversely• proportional

Example

• Playing middle C on a piano produces a sound with a frequency of 256 Hz. What is the period of the sound wave?

• f= 256 Hz• T=?• T=1/f• T=1/256• T=0.004 s

Speed of Waves

• Remember • V=d/t• So to get speed we need distance and

time• A single wave

• Distance travelled is one wavelength, λ

• Time is one period, T• Velocity of a wave=wavelength x

frequency• v=λf

Example

• An air horn sound at a frequency of 220 Hz. If the speed of sound in air is 330 m/s, what is the wavelength of the sound wave?

Example• The distance between successive crest in

a series of water is 4.0 m, and the crests travel 8.6 m in 5.0 seconds. Calculate the frequency of a block of wood bobbing up and down on these water waves.

• λ= 4.0 m• d=8.6 m• t=5.0 s• f=?• V=λf• V=d/t• V= 8.6/5 • V=1.72 m/s • 1.72=4f

Waves

Interference

Interference

• Two different material objects can not occupy the same space at the same time

• Waves are not matter but they can displace matter

• Waves can occupy the same place at the same time

• When waves overlap it is called superposition

Constructive Interference

• Two waves superimposing that are on the same side of equilibrium

• They will enhance each other or add up

Destructive Interference

• Waves are on opposite sides of equilibrium• They will weaken each other or subtract•

Standing Waves

• A wave that remains in constant position• Can occur when a medium is moving in

the opposite direction or when two waves interfere with each other, or constructive and destructive interference

Reflection From a Boundary

• When waves hit a boundary the wave will be reflected

• If the boundary is fixed the wave will be reflected AND INVERTED

• If the boundary is free the wave will just be reflected

Standing Waves

• If a series of waves are sent along a string the reflected pulse will interfere with itself

• If the waves are sent at just the right frequency we will create a standing wave

• Maximum Wavelength on a standing wave is 2L

Antinodes and Nodes

• Areas of complete destructive interference have NO amplitude are nodes

• Areas of complete constructive interference have LARGE amplitudes are antinodes

Waves

Sound

Sound

• Begins with a vibrating object

• Is a longitudinal wave• The compression is

where density and pressure are at a maximum (crest)

• The rarefaction is the region are the minimum points (trough)

Sound Characteristics

• Needs a medium to travel• The higher the temperature the faster

• Higher temp=more energy=more vibrations

• The more dense the faster • Vibrations will occur more quickly if the

molecules are closer together• Pitch is determined by frequency• Volume is determined by amplitude

Clicker Question

• Which statement is true?• A) If a tree fell in outer space no one can

hear it • B) A dog whistle has produces sounds that

have a high amplitude• C) If you are at a concert all of the sound

waves have a high frequency• D) Sound travels faster in air than water

Clicker question

• Which medium would have faster sound waves?

• A) Gas (at 0 degrees)• B) Liquid (at 0 degrees)• C) Solid (at 0 degrees)

Minimum and Maximum Frequency

• Frequency pertains to pitch not loudness• Minimum- 20 Hertz

• Below- infrasonic waves• Maximum- 20,000 Hertz

• Above-ultrasonic waves

• What are the corresponding wavelengths if the speed of sound in air is 343 m/s?

The Doppler Effect

• When there is movement involved there is an observed change in frequency

• This can be seen with all types of waves

The Doppler Effect

• You hear a high pitch as an ambulance approaches you and as soon as it passes the pitch decreases suddenly

• Compared to the noise being right beside you• It has a higher pitch as it approaches

you• It is a lower pitch when it moves away

from you

The Red Shift

• Red light has the longest wavelength (colors)

• When we look at other galaxies we notice that their colors are shifted towards red

• This is due to the Doppler effect

• What does this indicate?• The universe is

expanding

Resonance

• Objects/Substances can have a natural frequency in which it will begin to vibrate when that frequency strikes the object

• If another object begins emitting that wave at that natural frequency then the second object will begin to vibrate at the same frequency

• Example opera singer shattering glass, musical cups

Standing Waves/Harmonics

• On a string standing waves are made

• b-1st harmonic • natural frequency

• c-2nd harmonic• 1st overtone• One octave higher

• d-3rd harmonic• 2nd Overtone

Waves

Electromagnetic Waves and Light

Types of Waves

• Mechanical Waves• Use a Medium• Traverse (ocean waves) and

Longitudinal (springs and sound)• Electromagnetic Waves

• Do not require a medium• Are transverse waves• Are formed when an electric field is

coupled with a magnetic field • Light, microwaves, radiowaves etc

Electromagnetic Spectrum

Visible Light

• White light is a mixture of different colors

• Red light has the longest wavelength (700 nm)

• Violet has the shortest (400 nm)

• ROYGBIV

EM Radiation

• Is emitted from energized matter• After energy is absorbed by matter it is

emitted as EM radiation

Speed of Light

• The speed of light=c• The speed of light is 3.0 x 108 m/s in a

vacuum• IF there is a medium the speed will

depend on the type of medium• Denser=slower

Example

• The sun is 1.50 x 108 km from Earth. How long does it take for the light from the Sun to reach us?

• d=1.50 x 108 km• d=1.50 x 1011 m• c=3.0 x 108 m/s • t=?• v=d/t

Example

• What is the frequency of red light if the wavelength is 700 nm?

• λ=700 nm• λ=7.0 x 10-9 m• c=3.0 x 108 m/s • f=?• v=λf• c=λf

Waves, Sound and Light

Snell’s Law

Wave Speed

• Wave Speed depends on the medium

• When waves travel from one medium into another the speed will change

• As the waves moves to more shallow water the waves slow down

Wave Speed

• Waves traveling perpendicular to the new medium (θi=0 or angle of incidence) continue in the same direction

• Velocity changes, but the frequency stays constant

• Soo.. Wavelength will change

• When waves are not perpendicular they will bend

Refraction

• Refraction-bending of a wave due to changing medium

• Optical Density- a measure of how difficult it is for light to travel in a substance

• Vacuum<air<water<glass<diamond

Angles

• Angle of Incidence• Angle between the incident

ray (original wave/ray) and the normal

• Angle of refraction• Angle between the

refracted ray and the normal

Snell’s Law

• When light travels from • Less dense to more dense

medium it slows down and bends towards the normal

• More dense to less dense medium it speeds up and bends away from the normal

Snell’s Law

• nisinθi=nrsinθr

• ni=index of refraction (first medium)

• θi=angle of incidence

• nr=index of refraction (second medium)

• θr=angle of refraction

IndexMedium n Medium n

Vacuum 1 Crown glass 1.52

Air 1.0003 Quartz 1.54

Water 1.33 Flint Glass 1.61

Ethanol 1.36 Diamond 2.42

Example

• A ray of light traveling in air strikes a block of quartz at an angle of 15 degrees. Find the angle of refraction. Draw a diagram.

Example

• A ray of light travels from underwater into air. It travels in the air at an angle of 65 degrees, find the incident angle. Draw a diagram.

Index of Refraction

• n=c/v• n=index of refraction• c=speed of light in vacuum 3.0 x108 m/s • v= speed of light in substance

Example

• What is the speed of light in water?• c=3.0 x 108 m/s • n=1.33• v=?• n=c/v

Total Internal Reflection

• When passing from a more dense to a less dense medium, light reflects away from the normal

• If the angle of large enough then the angle of refraction will be parallel to the medium boundary (θr=90 degrees)

Critical Angle

• Critical Angle- θi that results in θr = 90

• Total internal reflection- the wave does not pass into the next medium, occurs when θi>θc

Example

• Find the Critical angle for light traveling from water into air. Draw diagram

• ni=1.33

• nr=1.0003

• θr=90

• θc=? Or θi

• nisinθi=nrsinθr

Waves, Sound, and Light

Reflection

Reflection

• When a wave travels into a new medium some will be reflected back

• The amount of reflection will depend on different the media are

Law of Reflection

• Light reflected from a plane (flat) mirror follows this law

• Angle of Incidence= Angle of Reflection

• θi=θr

• These angles are measured from the normal, the line perpendicular to the mirror

Ray Diagram- Flat (Plane)Mirror

Regular Reflection Versus Diffuse Reflection

Regular• You can see a reflected

image

Diffuse

Does not reflect an image

Plane Mirror Ray Diagrams

• When you look at an image in a plane mirror it is• The same size• The same distance behind the mirror as

you are in front of it• Right side up and laterally inverted

• The reflected light has the same speed, wavelength and frequency as the incident light

Curved Mirrors

• Concave- curved inwards• Convex-curved outwards

Ray Diagrams

Convex Concave

Definitions

• Principal Axis- Line through the center of the mirror

• Center of Curvature (C)-center of spherical mirror (center of the circle if the mirror was a complete circle)

• Focal Point (F)- point where all light converges (all rays pass this point)

• Focal length (f) distance from F to mirror and from C to F

Find the focal point

Focal Length

• 1 = 1 + 1• f do di

• f=focal length• do = distance of object from mirror

• di= distance of image from mirror

• Can be used for mirrors and lenses

Example

• An object is 15 mm in front of a concave mirror, and the image is 4.0 mm behind the lens. What is the focal length of the lens?

• do=15 mm

• di = 4 mm

• f=?• 1 = 1 + 1• f do di

• 1/f=1/15 + 1/4• 1/f=.3166• f=3.15 mm

Diffraction and Polarization

• Diffraction- waves can bend around objects

• White light can be split into multiple colors, with a diffraction grating in a prism