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# Doppler effect

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### Doppler effect

1. 1. Doppler EffectDoppler Effect
2. 2. DEFINITIONDEFINITION  First explained in 1842 byFirst explained in 1842 by Christian DopplerChristian Doppler, the, the Doppler EffectDoppler Effect is the shiftis the shift in frequency and wavelengthin frequency and wavelength of waves which results fromof waves which results from a source moving witha source moving with respect to the medium, arespect to the medium, a receiver moving withreceiver moving with respect to the medium, orrespect to the medium, or even a moving medium.even a moving medium.
3. 3. DEFINITIONDEFINITION  In other words, theIn other words, the DopplerDoppler Effect refers to theEffect refers to the change in pitch of a soundchange in pitch of a sound due to the motion eitherdue to the motion either of the source or of theof the source or of the listener.listener.
4. 4. Moving SourceMoving Source  Sound moves equally in all directions from a source.Sound moves equally in all directions from a source. • Circular (or spherical) patternCircular (or spherical) pattern  If the source is moving the origin of successiveIf the source is moving the origin of successive circles moves.circles moves.
5. 5. CHARACTERISTICS OF THECHARACTERISTICS OF THE DOPPLER EFFECTDOPPLER EFFECT  If two objects are approaching each other, or ifIf two objects are approaching each other, or if an initial object is approaching a second standingan initial object is approaching a second standing object, the pitch is higherobject, the pitch is higher  If two objects are moving apart, or if an initialIf two objects are moving apart, or if an initial object is moving apart from a second standingobject is moving apart from a second standing object, the pitch is lowerobject, the pitch is lower
6. 6. SoundSound The Doppler effect causes the changing pitch of a siren. When a firetruck approaches, the pitch sounds higher than normal because the sound wave crests arrive more frequently. When the firetruck passes and moves away, you hear a drop in pitch because the wave crests are arriving less frequently. The Doppler Effect Note: The change in loudness is not the Doppler Effect! It is the shift in frequency!
7. 7. EQUATIONEQUATION  The perceived frequency (The perceived frequency (ff ´) is related to the´) is related to the actual frequency (actual frequency (ff00) and the relative speeds of) and the relative speeds of the source (the source (vvss), the speed (), the speed (vv) of waves in the) of waves in the medium by an equation:medium by an equation: ff22 = fo ( v/ v+ v= fo ( v/ v+ vss)) if the source is moving away from the observerif the source is moving away from the observer ff22 = fo ( v/ v- v= fo ( v/ v- vss)) if the source is moving toward the observerif the source is moving toward the observer
8. 8. EXPLANATIONEXPLANATION  The choice of using the plus (+) or minus (-)The choice of using the plus (+) or minus (-) sign is made according to the conventionsign is made according to the convention that if the source and observer are movingthat if the source and observer are moving towardstowards each other the perceivedeach other the perceived frequency (frequency (ff ´) is´) is higherhigher than the actualthan the actual frequency (frequency (ff00). Likewise, if the source and). Likewise, if the source and observer are movingobserver are moving away fromaway from each othereach other the perceived frequency (the perceived frequency (ff ´) is´) is lowerlower thanthan the actual frequency (the actual frequency (ff00).).
9. 9. Sample problemSample problem  A car travelling at 100 km.hr sounds its horn as itA car travelling at 100 km.hr sounds its horn as it approaches a hiker standing on the highway. If theapproaches a hiker standing on the highway. If the car’s horn has a frequency off 44o Hz and thecar’s horn has a frequency off 44o Hz and the temperature of the air is 0 C what is the frequency oftemperature of the air is 0 C what is the frequency of the sound waves reaching the hiker?the sound waves reaching the hiker?
10. 10. Activity # 7: Doppler ShiftActivity # 7: Doppler Shift  You are standing at a railway. A train approaching atYou are standing at a railway. A train approaching at 125 km/h sounds its whistle. If the frequency of the its125 km/h sounds its whistle. If the frequency of the its whistle s 442 Hz and the air temperature s 20 Cwhistle s 442 Hz and the air temperature s 20 C what frequency do you hear when the trainwhat frequency do you hear when the train approaches you? when the train has passed by you?approaches you? when the train has passed by you?  A car sounds its horn (502 Hz) as it approaches aA car sounds its horn (502 Hz) as it approaches a pedestrian by the side of the road. The pedestrianpedestrian by the side of the road. The pedestrian has perfect pitch and determines that the sound fromhas perfect pitch and determines that the sound from the horn has a frequency 520 Hz. If the speed ofthe horn has a frequency 520 Hz. If the speed of sound that day was 340 m/s how fast was the carsound that day was 340 m/s how fast was the car travelling?travelling?
11. 11. Sonic BoomSonic Boom  The speed of sound is calledThe speed of sound is called Mach 1.Mach 1.  Faster than that speedFaster than that speed causes a shock wave – acauses a shock wave – a sonic boom.sonic boom. next
12. 12. Shock WaveShock Wave  A moving source canA moving source can exceed the speed ofexceed the speed of sound.sound.  The sound wavesThe sound waves constructively interfere onconstructively interfere on a front at an angle to thea front at an angle to the motion.motion.  This is called aThis is called a shockshock wavewave..
13. 13. Police use the Doppler effect of radar waves to measure the speeds of cars on the highway. Radar waves are electromagnetic waves. Police bounce them off moving cars. A computer built into the radar system compares the frequency of the radar with the frequency of the reflected waves to find the speed of the car. The Doppler Effect
14. 14. LightLight The Doppler effect also occurs for light. • When a light source approaches, there is an increase in its measured frequency. • When it recedes, there is a decrease in its frequency. The Doppler Effect
15. 15. Increasing frequency is called a blue shift, because the increase is toward the high-frequency, or blue, end of the spectrum. Decreasing frequency is called a red shift, referring to the low-frequency, or red, end of the color spectrum. Distant galaxies show a red shift in their light. A measurement of this shift enables astronomers to calculate their speeds of recession. The red shift Is also a piece of evidence for the Big Bang theory. The Doppler Effect
16. 16. When a source moves toward you, do youWhen a source moves toward you, do you measure an increase or decrease in wave speed?measure an increase or decrease in wave speed? The Doppler Effect
17. 17. How does the apparent frequency of waves change as a wave source moves? The Doppler Effect

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