WAVES
Newtonian Question
c=f*λ f=1/T
• An electromagnetic wave travels from
Atlanta to L.A. in 0.02154 s (one complete
cycle). How far is L.A. from Atl. in
Kilometers? (all EM waves travel at
3.0*10^8 m/s; 1000 m=1km)
•
•
•
•
A) 6,462,000 km
B) 1.39*10^10 km
C) 1.39*10^7 km
D) 6,462 km
The Doppler Effect
• The apparent change in frequency due to
the motion of the source of a wave (or its
receiver) is called the Doppler Effect
– Blue Shift – Increase in frequency
– Red Shift – Decrease in frequency
The Doppler Effect
The Doppler Effect
The Doppler Effect
What is a wave?
A wave
 is caused by a vibration
 is a wiggle in space and time
 carries energy not matter
1st Major Category
Mechanical Waves
 waves that require a medium (solid, liquid, or
gas) through which to travel.
 Speed of the waves depends on the elasticity of
the medium
 Are governed by Newton’s Laws
 3 types of mechanical waves: transverse,
longitudinal, and surface
Types of mechanical waves
• Transverse wave- particle displacement is
perpendicular to the wave motion
• Examples include waves in piano and
guitar strings
Types of mechanical waves (cont)
• Longitudinal waves- particle displacement
is parallel to the wave motion
• Sound is a longitudinal wave
• Fluids, liquids, gases, and plasmas usually
only transmit longitudinal waves
Types of mechanical waves (cont)
• Surface waves- a combination of both
transverse and longitudinal waves
• Waves on the water’s surface are
examples of surface waves
Properties of mechanical waves*
Crest- the high point of a wave
Trough- the low point of a wave
Amplitude- measurement of the amount of
energy the wave carries
*Some properties are only found in transverse waves and some
properties are only found in longitudinal waves.
Properties of mechanical waves*
(cont)
Wavelength- the distance between the
midpoint of one crest/trough to the
midpoint of the next crest/trough
Line of equilibrium- the resting point of a
wave
Draw the first type of mechanical wave, a
transverse wave, and label the crest, trough,
wavelength, amplitude, and line of equilibrium
Check your answer!
Draw the second type of mechanical wave, a
longitudinal wave, and label the compression,
rarefaction, and wavelength.
Check your answer!
Velocity of mechanical waves
• Calculated using the formula
• V = fλ
• Where v is velocity, f is frequency, and λ
(Greek letter lamda)is wavelength.
• Other variations:
»f=V/λ
»λ=V/f
Other relevant formulas
• T = 1/f
• f = 1/T
– Where T is the period and f is frequency
• Frequency is measured in HERTZ
– Hertz are literally per-seconds
– How ever many waves pass a certain point
PER SECOND is the frequency.
Solve the following problem
• Problem: Calculate the velocity of waves
in water that are 0.4 m apart and have a
frequency of 2 Hz.
Answer:
• v = fλ
• v = (2 Hz)(0.4 m)
• v = 0.8 m/s
One more try!
• Problem: What is the wavelength of a
340 Hz sound if the velocity of sound is
343 m/s?
Answer:
• v = fλ
• 343 m/s = (340 Hz)λ
• ~1.0 m = λ
Wave Interference
• Waves have a very unique property, they
can occupy the same space at the same
time.
• This phenomenon is called interference!
• Interference can be either constructive or
destructive.
Wave Interference (Cont)
• Constructive interference, or
reinforcement, occurs when the crests of
two waves overlap each other and the
amplitudes are added together.
Wave Interference (cont)
• Destructive interference, or cancellation,
occurs when a crest and a trough of two
waves overlap and their amplitudes cancel
out.
2nd Major Category
• Electromagnetic waves
Wave does not need a medium to travel
Wave that is partly electric and partly
magnetic
All EM waves are located on the
electromagnetic spectrum
All EM waves travel at the speed of light
(3.0 x 108 m/s)
The Electromagnetic Spectrum
Solving an EM problem
• The velocity for all EM waves is 3.0 x 108
m/s and is represented by the letter c.
• Therefore, the equation
v =fλ
becomes
c = fλ!
Solving an EM problem (cont)
• Problem: The wavelength of yellow light is
556 nm. What is the frequency?
• Answer: c = fλ
3.0 x 108 m/s = f(556 nm)
5.4 x 1014 Hz