grade 8 waves
TRANSCRIPT
WavesDefinition, behavior,
and classification
What is a wave?
A WAVE is a vibration or disturbance
in space.
What is a wave?
Waves transfer energy without
transferring matter.
How are waves classified?Waves can be
classified by WHAT they
move through or by HOW
particles move through them.
What material do waves move through?
A MEDIUM is the substance that
waves travel through and need to have in order to
move.
Classification of WavesAccording to what
they move through (Medium)
• Electromagnetic waves
• Mechanical Waves
Electromagnetic Waves
• waves that can travel through matter or empty space where matter is not present.
• radio waves• microwaves• infrared waves• visible light• ultraviolet rays• X-rays
Electromagnetic Waves
Mechanical Waves
• Needs a medium• require the particles of the medium to vibrate in order
for energy to be transferred.
• water waves• earthquake/seismic waves
• sound waves• waves that travel down a rope or
spring
Mechanical Waves
Classification of Waves
According to how particles move through them
• Transverse waves• Longitudinal
Waves
Transverse Waves
Displaces particles
perpendicular to the motion of
the wave
science.hq.nasa.gov/kids/imagers/ems/waves3.html
Longitudinal WavesCause the particles
of a medium to move parallel to the direction of
motion of the wave.
Longitudinal Waves
Has a crowded area causing a high-pressure region
called compression, and the opposite is
rarefaction
http://www.lcse.umn.edu/specs/labs/glossary_items/rarefaction_spring.html
Transverse and Longitudinal Wave
Which is which?
Wave Characteristics
Wavelength (λ)The distance from crest to crest (or trough to trough); expressed
in meters
Wave Characteristics
Amplitude (A)
The distance of crest (or trough) from the midpoint of
the wave
Wave Characteristics
Frequency (f)The number of waves that
passed a fixed point per second; measured in hertz (Hz)
Periodic Wave Characteristics
Period (T)The time it takes a wave to travel a
distance equal to a wavelength; measured in second
T= 1/f
Periodic Wave CharacteristicsPeriod (T)
start
end
Period (T)
start end
Period (T)
endstart
Periodic Wave Characteristics
Wave velocity (v)
Distance travelled by a wave crest in one period
v= λ/T
Wave BehaviorWhat happens when…• A wave meets a hard surface like a wall?• A wave enters a new medium?• A wave moves around an obstacle?• A wave meets another wave?
Reflection• When a wave hits
a surface through which it cannot pass, it bounces back.
• All types of waves can be reflected.
Reflection• When a wave hits
a surface through which it cannot pass, it bounces back.
• All types of waves can be reflected.
Reflection
Reflection does not change the speed or
frequency of the wave, BUT the wave
can be flipped upside down!
Refraction• The bending of a wave as
it enters a new medium.• It is caused by a change in
the speed of the wave as it moves from one medium to another– Greater change in
speed = more bending of the wave
Refraction
Refraction
DiffractionThe bending of a wave as it moves
around an obstacle or
passes through a narrow opening.
Diffraction• The scattering of a
wave as it hits the edges of a boundary or tiny opening.• The wave will try to
curve around the boundary or outward through the opening.
Refraction vs. Diffraction• Both cause bending,
but refraction occurs when waves move through something, while diffraction occurs when waves pass around something.
Interference
When two or more waves are moving through a medium at the same time,
they may overlap or combine together.
Interference
Constructive Interference
When two waves combine to make a wave with larger displacement or amplitude.–“Helping each other”
Destructive Interference
• Two or more waves combine to produce a wave with a smaller displacement.
Problem Solving Time! 1. A wave in a spring has a
wavelength of 0.1 meters and a period of 0.2 seconds. What is the speed of the wave?
• v= λ/T• v = 0.1 m / 0.2 s• v = 0.5 m/s
Problem Solving Time! 2. Find the wavelength of a wave in a
rope that has a frequency of 2.0 Hz and a speed of 0.4 meters/second.
• v= λ/T; T = 1/f• λ = v / f• λ = 0.4 m/s / 2.0 Hz• λ = 0.2 m
0 cm
5
-10
-5
103. What is the amplitude of
the wave?
Amplitude = 5cm
Problem Solving Time! 4. A hiker shouts on top of a mountain
toward a vertical cliff, 688m away. The echo is heard 4s after.
a. What is the speed of sound?b. The wavelength of the sound is 0.75m.
What is its frequency?c. What is the period of the wave?
Problem Solving Time!
a)Speedv = d/tv = 688m/2sv = 344m/s2 s
2 s
4 seconds
Problem Solving Time! b) Frequencyv = λ/T; f = v/λf = 344m/s / 0.75mf = 458.67 Hz
4 seconds
Problem Solving Time! c) PeriodT = 1/fT = 1/458.67 HzT = 2.18x10-3s
4 seconds
Problem Solving Time! 5. If the frequency of a sound wave is
doubled, what happens to the period of the same sound wave?
The period will be halved.
Problem Solving Time! 6. If the speed of wave is doubled,
what happens to the frequency of the same wave if the wavelength is constant?
The frequency will be doubled.
Problem Solving Time! 6. If the speed of wave is doubled,
what happens to the frequency of the same wave if the wavelength is constant?
The frequency will be doubled.
Problem Solving Time! 7. If the wavelength is halved and the
frequency is quadrupled, what will be the speed of the wave? (in multiples of v)
2V.
Problem Solving Time!
C
F
E
A
B D