waves, sound and light wave properties. what is a wave? disturbance or vibration that transmits...
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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