Doppler Effect
Students will learn about the
Doppler Effect
Doppler Effect (J. C. Doppler, 1842)
• Applies to all waves
• If there is motion between the source and the
observer.
• If the distance between the source and the
observer decreases, the “perceived”
frequency increases (higher pitch).
• If the distance between the source and the
observer increases, the “perceived” frequency
decreases (lower pitch).
Sound Example
• Police siren at 1600 Hertz increases frequency
as it approaches (higher pitch).
• Police siren at 1600 Hertz decreases frequency
as it leaves (lower pitch).
Light Example
• Many Galaxies are moving away from us and
from each other. This is evidence of the Big
Bang.
• As galaxies move away from us, the distance
increases, so the frequency of the emitted
light decreases. This is called a “color shift” or
“red shift” since light waves are shifted to
lower frequencies toward red. Atomic
emission spectra are “red-shifted.”
Fast Sources
• If the source itself of the wave is traveling fast,
the edges of the trailing waves construct a
“shock wave.”
• Example: jet flying faster than sound creates
shock waves that result in “sonic boom.”
Mach 1 is speed of sound; Mach 2 is 2X; Mach
3 is 3X, Mach 4 is 4X, etc.etc.
• Example: boats going faster through water
than water waves create bow waves (V wake)
Standing Waves
• If a wave pulse travels to a barrier and reflects
off the barrier such that the pulse and the
reflection have the same frequencies and
amplitude, but they are traveling in opposite
directions. The product wave will appear to
“stand still” in the horizontal direction.
• Pulse crests and reflected crests “line up”
• Pulse troughs and reflected troughs “line up”
Resonance
• Resonance is the natural standing wave
pattern established by an object.
• Depend on the physical dimensions of the
object. A small object does not have the low
resonant frequencies as does a large object.
(violin versus cello).
Example: guitar box, violin box, flute, piano
sound board, drums, wind instruments
Destructive Resonance
• 1. Shatter a crystal goblet by singing a note of
the right frequency at full voice.
• 2. November 7, 1940 Tacoma Narrows
Bridge. Wind gusts set up the span at its
natural frequency so that it twisted apart.
• 3. 1989 California Earthquake caused the
Oakland freeway collapse.