Physics 11 n Waves 6 – Doppler Effect

Constructive & Destructive Interference:

When waves meet:

Standing Waves:

Waves and Wave Length: n The standing wave to the left represents one half of the wavelength of the wave or ½. n This would be a complete wave cycle or 2/2 or 1. n This would be 3/2 or 1 ½ wave.

Fundamental Frequencies:

Interference is a characteristic of all waves.

Standing Waves n n When two sets of waves of equal amplitude and wavelength pass through each other in opposite directions, it is possible to create an interference pattern that looks like a wave that is “standing still.” It is a changing interference pattern. n Demo - Rope and strobe n Demo - Mechanical overhead model

Visible Spectrum:

θ Sonic boom manifested by condensation of water in air Sonic Boom: sin  = v sound /v source = 1/“Mach number”

Sonic boom of bullet in flight (holographic interferometry?) Sonic Boom: n How fast is the bullet traveling?  Mach # = 1/sin 

Super Sonic Flight: n _resources/boom_script_flash.html _resources/boom_script_flash.html _resources/boom_script_flash.html

DOPPLER EFFECT n n Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves n Doppler Movie - URL URL n Doppler Movie - URL URL

When a source of waves and an observer of waves are getting closer together, the observer of the waves “sees” a frequency for the waves that is higher than the emitted frequency. When a source of waves and an observer of waves are getting farther apart, the observer of the waves “sees” a frequency for the waves that is lower than the emitted frequency.

n All waves exhibit the Doppler effect. n n A particularly interesting example is used by astronomers to determine if light emitting objects (such as stars) are getting closer to us or farther away. n n On average most stars are moving farther away, and their light spectra are “red shifted.”

Red Shift: Moving Away from source Actual Visual Spectrum Observed Visual Spectrum

Blue Shift: Moving towards source Actual Visual Spectrum Observed Visual Spectrum

n Radar bounced off a spinning planet can exhibit a Doppler effect and lead to a determination of the spin rate of the planet. n This was used to discover that Venus has a retrograde spin. n Of course police use the Doppler effect to catch speeding motorists.

BOW WAVES n n Waves in front of moving object pile up. n n The familiar bow wave generated by a speedboat knifing through the water n n It is a non-periodic wave produced by the overlapping of many periodic circular waves. It has a constant shape.

Video Demonstration: n Demo 2.1 – Waves & Wake from Phys123

SHOCK WAVES n n Just as circular waves move out from a swimming bug, spherical waves move out from a flying object. If the object flies faster than the waves, the result is a cone-shaped shock wave. n n There are two booms, one from the front of the flying object and one from the back.

Video Demonstrations:

- slower than the speed of sound n Subsonic n Supersonic - faster than the speed of sound n n Mach Number = speed of sound speed of object