The Doppler Effect describes how the frequency of waves is altered by the motion of the source or observer. It was first explained in 1842 by Christian Doppler. When the source and observer are moving towards each other, the perceived frequency is higher than the actual frequency. When they are moving away from each other, the perceived frequency is lower. This shift in frequency due to motion is known as the Doppler Effect and applies to sound waves, light waves, and other wave phenomena.

1. Doppler EffectDoppler Effect

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

17. 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

18. How does the apparent frequency of waves
change as a wave source moves?
The Doppler Effect