Types of Waves
Physics 2A
Traveling Waves
and Sound
Types of Waves
Sound
The Doppler Effect
An Electromagnetic Wave
Mechanical Waves ⇒ waves that involve the
motion of a substance (the medium) through
which the wave moves
Electromagnetic Waves ⇒ a transverse wave
consisting of oscillating electric and magnetic
fields (no medium is required)
Matter Waves ⇒ at the atomic level, particles
such as electrons and atoms must be treated as
waves called matter waves
Transverse Wave
disturbance
⇒ An electromagnetic wave is a transverse
wave consisting of oscillating electric and
magnetic fields.
direction of travel
Longitudinal Wave
disturbance
Sound is a longitudinal wave
A wave in which the disturbance is parallel to the direction of
travel of the wave.
direction of travel
disturbance
direction of travel
Sound
Sound
⇒ In sound waves, air molecules are disturbed in the same
direction as the wave.
⇒ However, an individual air molecule does not travel
with the wave. It only oscillates back and forth.
⇒ All sound is produced by an
object that oscillates back and
forth.
Sound
Sound
⇒
A speaker produces sound by a diaphragm which
oscillates back and forth.
⇒
The diaphragm produces successive regions of
increased pressure (condensations) and decreased
pressure (rarefactions).
Regions of increased pressure are Regions of decreased pressure are
called condensations.
called rarefactions.
Sound
Sound
condensation
⇒ Your eardrum then detects the differences
in air pressure.
Sound
An Electromagnetic Wave
⇒ An electromagnetic wave is a transverse
wave consisting of oscillating electric and
magnetic fields.
The Visible Spectrum
The Electromagnetic Spectrum
⇒ Imagine if you can the world view of little
creatures who could see only a very tiny portion
of the visible spectrum, creatures who are color
blind to all other parts. Their world view would
be very limited.
⇒ Guess what? We are like those little creatures,
in that the spectrum of colors we can see are a tiny
portion of the electromagnetic spectrum!
⇒ The electromagnetic spectrum consists of
electromagnetic waves of all frequencies.
The Electromagnetic Spectrum
A stationary sound source
⇒ Visible light makes up a very small portion (less
than one millionth of 1%) of the measured
electromagnetic spectrum.
If the sound source is moving toward an observer,
the observer will hear a sound of higher frequency.
I hear a
lower frequency
What happens if the sound source is moving?
The Doppler Effect
I hear a
higher frequency
If the sound source is moving away from an observer,
the observer will hear a sound of lower frequency.
⇒ The frequency of sound (pitch) increases when
a source moves towards you.
⇒ The frequency of sound (pitch) decreases when
a source moves away from you.
The Doppler Effect
The Doppler Effect:
Formal definition: the change in frequency or
pitch of the sound detected by an observer
because the sound source and the observer have
different velocities with respect to the medium of
sound propagation.
⇒When an observer is moving towards a stationary source, the
frequency is greater (and the wavelength is smaller).
⇒When an observer is moving away from a stationary source, the
frequency is smaller (and the wavelength is bigger).
An object traveling at the speed of sound
What it means: if either you or the source of
sound is moving, you will hear a sound of a
different frequency than is being emitted.
Bow Waves and Shock Waves
bow wave ⇒ V-shaped wave produced by an object
moving on a liquid surface faster than the wave speed.
⇒ A similar thing happens when a plane is traveling
A shock wave is formed!
faster than the speed of sound. A sonic boom is heard
when the shock wave reaches listeners on the ground.
Breaking the Sound Barrier
sin θ =
v
vs
v = speed of sound
vs = speed of object
We could figure out the speed of the jet by measuring
the angle of the Mach cone!