Wave Notes
I. WAVES—a disturbance that transfers energy through
matter or space.
A. Energy Transfers: If a rock is
dropped into the water, the
kinetic energy (KE) is transferred
to the water by causing the
particles of water to move which
causes the neighboring particles
of water to move. In this way,
energy is transferred from one
place to another.
B. Examples of Energy Transfer:
1. A rolling bowling ball (moving matter) transfers
energy from your hand to the pins. This is NOT a
wave.
2. If the pins were lined up down the alley
like dominos and the energy from your
hand caused the first to move, the other
pins would in turn fall. This IS a wave.
II. MEDIUMS—substance or region through which a wave is
transmitted.
A. A medium transfers wave
energy but has no overall
motion itself.
B. Energy is transmitted
from one place to
another but matter does
NOT move between
these places.
C. Examples: What is the medium/matter?
1. Waves that can only travel through
matter are called mechanical waves.
a. Ocean waves have water for a
medium.
b. Sound waves have air for a
medium.
2. Electromagnetic waves do NOT require a medium!
They can be transmitted through a vacuum (space).
(Remember: Heat transfer—RADIATION.)
III. TWO TYPES OF WAVES
A. Transverse—waves in which the motion
of the medium is at right angles to the
direction of the wave.
Motion
Examples:
1. ocean waves
Direction of Energy
2. Light
3. A cork or a bobber on a fishing line moves up
and down as a wave passes because the
medium (water) is moving up and down as
the energy is transferred.
B. Longitudinal or Compression—waves consist of a series of
compressions and rarefactions.
Molecules of the medium move in the same direction or parallel
to the direction of the wave.
* Think of a slinky being pushed or pulled.
• Compression–a space in a medium in which molecules are
crowded together
• Rarefaction–a space in the medium where there are fewer
molecules
• Examples: Sound waves are longitudinal.
IV. THREE BASIC CHARACTERISTICS OF WAVES
A. Amplitude—maximum distance the molecules are
displaced from their normal rest position
Amplitude indicates the energy of the wave in a
transverse wave.
B. Higher energy is indicated by a
tightly compacted medium in a
longitudinal wave.
C. Frequency—number of complete
waves or cycles per unit of time
UNIT = Hertz (Hz) = 1 wave/sec
Example: If you move a jump rope
up and down twice in 1 second
the frequency would be 2 Hz.
V. SPEED OR VELOCITY OF WAVES (v = d/t)
A. Speed of a wave in a medium depends on the
properties (phase, density) of that medium.
The speed through a medium is constant.
B. Amplitude has NO effect on speed.
C. Speed (or velocity) is determined by the
number of waves passing in 1 second the
length of the wave.
Velocity (v) = Frequency (f) x Wavelength (λ)
D. Since speed (or velocity) is constant in a given
medium: if wavelength increases, frequency
decreases. (Inverse relationship)
λ↑ f↓ or λ↓ f↑
v=fλ
v=fλ
Example Problems:
1. A sound wave has a frequency of 110 Hz and a
wavelength of 3 meters. What is the speed?
Given: f = 110 Hz
Formula: v = f X λ
Solve: 110 Hz X 3 m = 330 m/s
λ = 3m
v = 330 m/s
2. A wave is traveling at a speed of 12 m/s and its
wavelength is 3 meters. Calculate the wave frequency.
Given:
Formula:
Solve:
3. A wave is traveling at a speed of 18 m/s and its
frequency is 3 Hz. What is the wavelength?
Given:
Formula:
Solve:
4. If a wave is traveling at a speed of 16 m/s and has a
wavelength of 4 meters, what is the frequency of this
wave?
Given:
Formula:
Solve:
Interactions of Waves
VI. Waves traveling in the same medium move at a
constant speed and in a straight line. However, waves
interact if the waves encounter a different medium,
obstacle or another wave.
1. Reflection—is the bouncing back of a
wave after it strikes a boundary that
does NOT absorb the wave’s energy.
• The incoming wave is called an
incident wave.
• The wave that is bounced back is
called a reflective wave.
• The line perpendicular to the barrier
is called the normal.
The Law of Reflection states that the
angle of incidence is equal to the angle of
reflection.
2. Refraction—is the bending of a wave due to a change
of speed as a wave passes at an angle from one
medium to another.
3. Diffraction—is the bending of waves around the edge
of a barrier or opening. It is a result of new series of
waves being formed when the original waves strike the
barrier of opening.
4. Interference—is a process that produces a new wave when
two or more waves arrive at the same point at the same
time. The two waves can combine in 2 different ways.
a. Constructive interference—occurs
when the crests of one wave meet
the crests of the other. The two
waves form together in a single
wave with an amplitude that is the
sum of the original waves.
b. Destructive Interference—occurs
when crests of one wave meet
troughs of another. The amplitude is
the difference between the
amplitudes of the original waves.
Sound Waves
VII. Sound—a form of energy that causes molecules of a medium
to vibrate back and forth
Series of compressions and rarefactions—longitudinal waves
***If there are no molecules, there is no sound.
1. SPEED of SOUND
a. In air, speed of sound is around 340 m/s.
b. Speed increases as temperature because the molecules of
the air are moving faster and bump into each other more
often.
c. Sound is transmitted faster in solids than liquids because
these materials are more elastic. (Elastic = can return to
their original shape) Sound is slowest in air.
2. INTENSITY and LOUDNESS
a. Intensity of sound is
determined by the
amplitude of the sound
wave. Intensity determines
loudness of sound.
b. Intensity is measured in
unit called decibel.
3. FREQUENCY and PITCH
a. Pitch depends on how fast the molecules of a
medium vibrate which depends on the frequency of
waves.
b. The number of waves passing a point in a second is
called cycles per second or Hertz (Hz).
High
Low
c. Human ear has a range of 20 Hz to 20,000 Hz.
d. The pitch (frequency) of a sound can change if the speed of the
sound changes.
Example:
• Your voice sounds higher pitched when you inhale helium
because the sound travels faster in the helium than air!
• Wind instruments can go “sharp” as the temperature increases
because the speed of sound increases. The wavelength (length
of air column) stays the same, so the frequency must change.
4. DOPPLER EFFECT
Doppler Effect—a change in frequency and pitch of a sound due
to motion of either the sound source or observer
• If source of sound moves toward observer, waves are generated
from points that are closer and closer.
• Therefore, waves reach the observer sooner. The observer hears
more waves/second, causing higher frequency (higher pitch).
• If source of sound moves away, waves reach observer later (fewer
waves/second), causing lower frequency.
Think about the following question?
What does a fire truck siren sound
like? Does it sound different when it
is passing you by on the road?
5. QUALITY OF SOUND
Timbre—quality of sound caused by blending of pitches
Example: A tuning fork has a pure tone (one frequency). Voice,
strings, air columns produce tones that are blends of pure
tones that account for the difference in quality. This is why 2
different instruments can play the same pitch and sound
different.
6. RESONANCE
Every medium has its own frequency of vibration called natural
frequency.
Resonance—ability of an object to vibrate by absorbing energy
in its natural frequency
Examples:
• The vibrations of a sound board of a violin
• The note you get when you blow into a half
full bottle of soda
• Singer breaking a glass
7. CONSTRUCTIVE and DESTRUCTIVE INTERFERENCE
• Sound waves can combine constructively and destructively.
When in phase the intensity of sound is increased. When
out of phase, sound is softer and “dead spots” can occur.
• Beats—phenomenon caused by
combination of 2 waves of slightly
different frequencies. This causes
intensity to vary.
Light Waves
VIII. Light is both a wave and particles. This contradiction
perplexed scientists for many, many years. The speed of light
is 3 x 108 m/sec. Sound is fast (340 m/sec), but light is faster.
Light can circle the earth 27 times in one second. Scientists
now believe that nothing can go faster than light.
Question: Why do you think you see a flash of lightning before you hear a clap of thunder?
2. VISIBLE LIGHT
What we call “visible light” is made up of many different colors.
Each color has a different wavelength and a different frequency.
A. A prism uses refraction to separate the different wavelengths
(colors) of visible light.
Red → long waves to short waves → Violet
B. Colors have different energies. You know that different flames
give off different amounts of heat. Red flames are the coolest
and blue flames are the hottest. As you move from red to blue,
light GAINS energy. White light is made up of all colors—that is
why a white flame is the hottest!
Which color has the longest wavelength? Red
Which color has the shortest wavelength? Violet
Which color has the highest energy? Violet
C. Visible light is a very small part of the entire electromagnetic spectrum.
• Radio waves—used to transmit radio and television signals.
• Microwaves—used to cook food and by cell phones.
• Infrared—invisible heat
• Visible (white) light
• Ultraviolet light—invisible wavelengths; part of sunlight burns your skin and can
cause cancer. The ozone layer protects us from most of the sun’s ultraviolet light.
• X-rays—used in medicine and industry.
• Gamma rays—the most powerful and dangerous form of radiation. Emitted by
nuclear reactions, they can break chemical and nuclear bonds.
3. LENSES and MIRRORS
Lenses and mirrors work
opposite of each other. If a
concave lens reduces, then a
concave mirror magnifies.
A. Concave and Convex
• Concave looks like the sides have
caved in.
• Convex—the middle is bigger
than the ends.
B. Lenses—work by refraction, by
the light bending when moving
between two substances.
C. Mirrors—work by reflection, by the bounding of
light off of a shiny surface. Images in mirrors
always look twice as far away as the object.
Seismic Waves
IX. The movement of Earth’s plates creates stress in the
rock. When the stress in the rock builds up enough,
the rock breaks or changes shape, releasing energy
in the form of waves or vibrations. The waves
produced by earthquakes are known as seismic
waves. (The word seismic is Greek for EARTHQUAKE.)
4. Seismic waves include primary waves,
secondary waves and surface waves.
A. Primary Waves—are longitudinal waves or P
waves. They move faster than other seismic
waves and so arrive at distant point before
other seismic waves.
B. Secondary Waves—are transverse waves or S
waves. Secondary waves CANNOT travel
through liquids. Since part of the earth is
liquid, S waves do NOT travel directly through
Earth and cannot be detected on the other
side of Earth.
C. Surface Waves—P and S waves can be
transformed into surface waves at Earth’s
surface. Surface waves are a combination of
longitudinal and transverse waves. Even
though surface waves travel more slowly than
P or S waves, they produce the most severe
ground movements.
5. Seismographs—detect and measure earthquake waves. A
seismograph records the ground movements caused by seismic
waves as they move through the Earth.
6. Tsunami—a seismic
wave or tsunami is a
devastating water
wave generated by
an undersea
earthquake.
F/A-18 Hornet passing through the
sound barrier. Photograph taken by
Navy Ensign John Gay while aboard
the carrier USS Constellation,
07/07/99.
Sonar used to find sunken ship.
Your first baby picture was
taken using sonar.