IB Physics Doppler effect flippingphysics by Nothingnerdy
presentsa production Doppler Effect 1
Doppler effect Source stands still and emits waves all around it. All observers experience the same frequency. Source travels to the right. Waves emitted forward have a shortened wavelength; waves emitted behind have a lengthened wavelength. Images: Bauer
Moving observer Stationary source ObsThe moving observer will detect waves with a shorter wavelength because it is moving in the opposite direction to the waves. Images: Bauer
Moving source Stationary observer Obs ObsThe green observer will detect waves with a shorter wavelength because the source is moving towards it. The red observer will detect longer waves. Images: Bauer
Doppler sound The sound of a moving car is different depending on whether the car is approaching the observer or receding. short wavelength = high frequency = high-pitched soundA moving observer also experiences a higher sound ifshe is approaching a stationary source. eg an observer on a train passing a ringing bell.
Doppler calculation f’ = frequency detected f = frequency emitted v = velocity of sound uo = velocity of observer us = velocity of sourceWhen the velocities are in opposite directions, we add them. A police car siren emits a Speed of sound = 330 m/ssound of frequency 600 Hz A cyclist approaches at 18while travelling at 40.0 m/s. m/s a stationary transformerWhat does this sound like humming at 50.0 Hz. What to a stationary observer wavelength of sound does behind the car? [535 Hz] she measure? [6.26 m]
Red shiftThe absorption linesin the spectrum ofstars are distinctive.If a galaxy is movingwith great speed, wecan measure the shiftof the lines. If it isapproaching, thefrequency is higher(blue-shift). If the This is the method whichgalaxy is receding Edwin Hubble used to establish(most are), the lines that the universe is expanding.are red-shifted. Image: sciencephoto.com
Calculating red shift ∆f = change in frequency v = relative speed of source and observer An approximation of c = speed of light (300 000 km/s)the Doppler formulas f = original frequency when c >> v A star emits light at 456 THz which is detected at the earth as 416 THz. What is the speed of the star relative to the Earth? [26 300 km/s] Infra-red radiation from a police radar gun bounces off a car travelling at 30 m/s. The car receives a frequency of 200 THz; what will be the change in frequency? [20 MHz]
Using Doppler shift The speed of blood ﬂow can be measured using Doppler shift of ultrasound. In this application, like theradar speed trap, there is adouble Doppler shift: once when the wave hits the object and again when it is reﬂected. Image: photolibrary
The background photo shows 1.5 million galaxies artiﬁcially coloured according to their Doppler shift. Red is the greatest shift (thus most distant). The Milky Way stretches across the middle of the map.
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