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 California Earthquake caused the Oakland freeway collapse.