POPS Unit 3 Waves - Teacher Notes
What is a wave? - A disturbance that carries energy through matter or space.
Both particles and waves carry energy
Particles:
Carry energy
Transfer matter
Example: Throwing a ball to a friend
Waves:
Carry energy
Transfer matter
Example: Throwing a ball to a friend
Carry energy
Do not transfer matter
Example: Shaking one end of a rope carries energy. Rope remains in your hand.
Medium - Any kind of material or space through which a wave might travel.
Wave Pulse -a single bump or disturbance that travels through a medium
Oscillation - A repeating cycle
The Anatomy of a Wave
Parts of a wave – overview
Crest: A wave’s high point, as measured from the midpoint
Trough: A wave’s low point, as measured from the midpoint
Wavelength: Distance from one wave to another.
ex: peak to peak, trough to trough
𝑣
λ=𝑓
Amplitude (displacement):
• The distance from the midpoint to the crest
(or trough) of the wave.
•
Represents loudness, brightness of color
• Greater amplitude, greater energy transmitted (work performed)
Node: Points of no displacement that appear to be standing still.
Anti-node: Points that undergo maximum displacement during each oscillation.
Describing Waves -Period and Frequency
Two ways to talk about wave movement:
1. Period:
 Tells how long it takes for 1 wavelength to go by. (seconds/oscillation)
2. Frequency:
 Tells how many waves go by in 1 second. (oscillations/second)
Period and Frequency
Period and frequency are reciprocals
Don’t write “oscillations.”
The unit for period is: s
The unit for frequency is: 1/s - Hertz (Hz)
Frequency:
How many waves go by in 1 second?
1
T=𝑓
1
𝑓=𝑇
Relationship between Wavelength and Frequency – Inverse relationship
Longer λ
Lower f
Shorter λ
Higher f
Speed (s):
The distance traveled by a given point on the wave (such as a crest) in a given interval of time (m/s).
distance
v=
time
Speed depends only on medium
The speed of sound in air is 340 m/s
Types of waves - Mechanical Waves and Electromagnetic waves
Mechanical Waves
Mechanical waves require a medium like water, air, ropes, spring
3 typ
Because electromagnetic waves cannot be directly observed, mechanical waves are good model for study.
Types of Mechanical Waves ,Transverse waves, Longitudinal waves, Surface waves
Longitudinal waves
Examples: - Sound waves
A longitudinal wave is one that vibrates parallel
to the direction of the wave’s motion
Fluids usually transmit only longitudinal waves
Transverse waves - A transverse wave is one that vibrates perpendicular
to the direction of the wave’s motion.
Examples: Light and seismic (S) waves
Surface Waves: - A combination of longitudinal and transverse waves
Electromagnetic waves
-Transverse wave
-Cannot be directly observed
-Travel through space at speed of light (c).
- c = 299,792,458 m/s
Examples:
-Light
-Radio waves
-X-Rays
0 Both Magnetic and Electrical Properties
-Behave as Both Waves and Particles
-Light behaves as a transverse wave which we can filter using polarized lenses.
-Light behaves as a particle that can knock electrons off of a substance.
-Photoelectric effect
All Forms of Light are Identical Except for Their Wavelength
Long Wavelengths
-About a mile
-Radio Waves
Short Wavelengths
- Atomic nuclei
-Gamma rays
The Electromagnetic Spectrum
Radio Waves
-Longest wavelength
-Lowest frequencies
-GPS
-Radio
-Each radio station broadcasts at a different frequency
-Number on dial is frequency.
-Cell Phones
Microwaves
Compared to radio waves: Shorter wavelength and higher frequency and more energy
Can penetrate haze, light rain and snow, clouds and smoke.
-Good for viewing the Earth from space.
-Longer microwaves heat our food in a microwave oven.
-Closer to a foot in length
-Shorter microwaves used for radar
-Just a few inches long
-Used in weather forecasts.
Infrared Waves
-Compared to micro waves
-Shorter wavelength, Higher frequency, More energy
-Longer wavelengths feel warm on your skin
-Warm objects give off more heat energy than cool objects.
-Therefore people give off infrared rays.
Visible Light
-Compared to infrared rays, Shorter wavelength, Higher frequency, More energy
-The only electromagnetic waves we can see
Ultraviolet Waves
-Compared to visible light, Shorter wavelength, Higher frequency, More energy
-Used to kill bacteria.
-Sterilization of equipment
-Too much can cause skin cancer.
-Use sun block to protect against UV rays
X- Rays (Waves)
-Compared to ultraviolet waves, Shorter wavelength, Higher frequency, More energy
-Can penetrate most matter.
-Bones and teeth absorb x-rays.
-The light part indicates where x-ray was absorbed
-Too much exposure can cause cancer
-Lead vest at dentist protects organs from unnecessary exposure
-Used by engineers to check for tiny cracks in structures.
-Rays pass through the cracks
-Cracks appear dark on film.
Gamma Rays
-Compared to X-Rays ,Shortest wavelength, Highest frequency , Most energy
-Wavelengths so short, they can pass through the space within atoms of a detector.
-Used in radiation treatment to kill cancer cells.
-Can be very harmful if not used correctly.
-Exploding nuclear weapons emit gamma rays
Digital and Analog
What’s Analog?
-A wave is recorded or used in its original form.
-Examples Thermometer, Record players
What’s Digital?
-An analog wave is sampled at regular intervals
-Sample is turned into numbers
-Numbers are stored in the digital device.
-Examples: cd’s, computers, dvd’s
-Sampling rate on a CD is 44,000 samples per second.
Advantages and Disadvantages
Advantages of Digital
-Recording does not degrade over time.
-As long as the numbers can be read, you will always get exactly the same wave.
-Digital is significantly more compressible
-More data in less space.
-Endless application possibilities
Advantages of Analog
-Higher quality Digital devices translate and reassemble data and in the process are more prone
to loss of quality as compared to analog devices. This is a challenge of the digital movement
PART 2 Waves at Boundaries
Vocabulary
-Incident Wave: A wave that strikes a boundary
-Reflected Wave: The returning wave after an incident wave has struck a boundary
-Transmitted Wave: A wave passing through an interface to a new medium
-Interface: The boundary between two media (plural medium is media)
-Polarity: Refers to a wave’s “up-ness” or “down-ness.”
-Two waves can have the same or opposite polarities.
Waves at Rigid Boundaries
Waves at Soft Boundaries
Reflection, Refraction, Diffraction
-The Law of Reflection
-When a wave meets a boundary, there
will be some reflection off the boundary.
-Angle of incident ray is equal to reflected ray
0 (Ɵ_i =Ɵ_r)
Do not consider angles with respect to the surface
The Law of Reflection
-Angles are measured with respect to a line perpendicular to the
boundary.
-“The normal”
Refraction
-When a wave meets a boundary, there will be some reflection off the boundary.
-Angle of refraction usually does not equal angle of incidence
-(Ɵ_i ≠ Ɵ_t)
-Could they be the same?
Refractive Index
Materials are assigned a Refractive Index based on
-the speed at which light travels through them.
Diffraction
-A wave’s change in direction as it passes through an opening or around a barrier.
-Diffraction depends on wavelength and size of object.
Superposition - Two or more waves combine
-Interference
-Add or subtract depending on polarity.
Destructive Interference
-Equal amplitude, opposite polarity
-Combined amplitude is zero
-After collision, each wave resumes original form
Constructive Interference
- Equal amplitude, same polarity
-Combined amplitude is greater than either wave
-After collision, each wave resumes original form
Combinations of Interference
-When 2 waves don’t match perfectly in amplitude or phase.
Standing Waves and Resonance
- From Bridges to Violins
-Standing Waves (harmonics)
-A standing wave is the result of interference between two traveling waves moving in opposite
directions.
-The frequency of the nth harmonic is linear.
fn = nf1
-If f1 were 100 Hz, f2 would be 200 Hz, and f3 would be 300 Hz, and so on.
-Different materials have different frequencies at which they produce their standing waves.
0 For example, the frequency at which a string produces a standing wave depends on the
length of the string, its mass, and the tension it is under.
Resonance
-Forced oscillation that is “in tune” with the natural oscillation frequency of a system.
-This is when the “beats” disappear – the two tones “resonate” instead of interfere with each
other.
-Everything has a resonance it “prefers.”
-When a forced oscillation is “in tune” with the preferred frequency of the object, it will
resonate.
The Doppler Effect
-Wavelengths are shortened or lengthened depending on whether the source is moving toward
or away from the receiver.
A Stationary Sound Source
-Sound waves are produced at a constant frequency.
-The wavefronts propagate away from the source at a constant speed.
-The distance between wavefronts is the wavelength.
-All observers will hear the same frequency, which is equal to the actual frequency of the source.
-How fast do sound waves travel through air?
A Source Moving Less Than the Speed of Sound
-The source is moving, so the center of each new wavefront is now slightly displaced to the right.
-As a result, the wavefronts begin to bunch up in front of the source, and spread further apart
behind the source.
-The wavelengths in front are shorter
-The wavelengths in back are longer.
What if the Source Moves Faster than the Speed of Sound?
-Since the source is moving faster than the sound waves it creates, it actually leads the advancing
wavefront.
- The sound source will pass by a stationary observer before the observer actually hears the
sound it creates
-The pressure front is intense (a shock wave), due to all the wavefronts adding together.
Red Shift and Blue Shift
-Because wavelengths toward the “blue” end of the visible spectrum have shorter wavelengths,
shortened wavelengths in the front of a moving object are said to be “Blue Shifted.”
-Lengthened wavelengths behind the source are said to be “Red Shifted.”