Doppler effect

1. Lesson #8Lesson #8
Doppler Effect
Nelson Reference: 433 - 435Nelson Reference: 433 - 435

2.  The following video demonstrates the DopplerThe following video demonstrates the Doppler
effect.effect.
 http://www.youtube.com/watch?v=ZPJyYaXhuv4&feature=relatedhttp://www.youtube.com/watch?v=ZPJyYaXhuv4&feature=related
 Named after Christian Doppler, an AustrianNamed after Christian Doppler, an Austrian
physicist (1803-1853).physicist (1803-1853).
 Why do we hear this effect? The diagrams (nextThe diagrams (next
slide) indicate what happens to the soundslide) indicate what happens to the sound λ. In. In
the first diagram,the first diagram, λ does not change – this isdoes not change – this is
seen by the distance from one circle (wave front)seen by the distance from one circle (wave front)
to the next being a constant value. However,to the next being a constant value. However,
when the source starts to move, and thewhen the source starts to move, and the
observers are not moving,observers are not moving, λ does change.does change. λ A
is _________thanis _________than λ B . This means that the. This means that the
frequency heard byfrequency heard by A, is _____________ than, is _____________ than
the frequency heard bythe frequency heard by B, or, or fA ___ fB ..

3. Doppler Effect
SA B
O b s e r v e r s A & B a r e e it h e r :
a . ) A t r e s t w it h t h e s o u r c e ( S )
b . ) M o v in g w it h t h e s o u r c e
N o D o p p le r E f f e c t i s n o t ic e d
b y o b s e r v e r A o r B
SA B
O b s e r v e r s A & B N O T m o v in g
b u t t h e s o u r c e is m o v in g in t h e
d ir e c t io n s h o w n .
D o p p le r E f f e c t is n o t ic e d
b y o b s e r v e r A a n d B
D O M
o f S
BA

4. Source or Observers Can Move for
the Doppler Effect to Occur
 The Doppler effect does not require onlyThe Doppler effect does not require only
the source (the source (S) to be moving. The source) to be moving. The source
may be at rest but the observer is movingmay be at rest but the observer is moving
toward, or away from, the source. Thetoward, or away from, the source. The
equation on the next slide can be used forequation on the next slide can be used for
either of these situations.either of these situations.

5. Deriving the Doppler Frequency Equation
 vS is theis the source speed
 ΔΔd is the change ind is the change in λλ
ΔΔd =d = vvSS (1/f), f is for(1/f), f is for S
(when not moving)
 λλ ´ is the new´ is the new λλ ..
 Speed of soundSpeed of sound v, isis
constant at Tconstant at TCC, so:, so:
v = f λ = f´ λ ´= f´(λ +
Δd)
But, Δd = vS (1/f)
 So,So, v = f´(λ + vS (1/f))
 Rearranging:
f´ = v / (λ + vS (1/f))
But, λ = v/f
So, f´ = v / (v/f + vS (1/f))
Simplifying:Simplifying:
f´ = (f v) / (v + vS ))
Here, we use “+” for a
lower f´ and “-” for a
higher f´
{This equation is the same as
given in your TB with the
speed of the detector = 0.}

6. Practice Questions
 The pitch of a car’s horn isThe pitch of a car’s horn is 520 Hz. If the car. If the car
travels by someone (who is not moving) attravels by someone (who is not moving) at 90
km/h and the air temperature isand the air temperature is 220
C, determine, determine
the frequency of the horn a.) when the carthe frequency of the horn a.) when the car
approaches the person b.) when the car drivesapproaches the person b.) when the car drives
away from the person.away from the person.
 Ans a.) 561 Hz b.) 485 Hz
Nelson TB:
Page 435 (Top) # 1,2
Page 435 (Bottom) # 2, 3, 4, Page 444 #43, 44

7. The following video shows how the speed of aThe following video shows how the speed of a
plane affects the sound waves coming from theplane affects the sound waves coming from the
plane. The video also shows the plane travelingplane. The video also shows the plane traveling
at the speed of sound and faster than the speedat the speed of sound and faster than the speed
of sound.of sound.
http://www.youtube.com/watch?v=-Zu5SGllmwchttp://www.youtube.com/watch?v=-Zu5SGllmwc