DOPPLER-EFFECT_20241016_201808_0000.pdf .pdf

Doppler Effect is a topic for Waves and Optics

1. Think About It
How does the sound of a
moving car's horn change as it
approaches and then passes
by you?

2. Answer Key
As the car passes and
moves away, the sound
waves are stretched out,
causing the pitch to
sound lower. This is
because the car is moving
away, increasing the
wavelength of the sound
waves.
Think About It
This change in pitch as
the car approaches and
passes is an example of
the Doppler effect, where
the motion of the sound
source affects the
frequency of the sound
you hear.
As a car approaches, the
sound waves from its horn
are compressed, causing
the pitch to sound higher.
This is because the car is
moving toward you,
shortening the
wavelength of the sound
waves.

3. DOPPLER
EFFECT
Exploring the Shift in
Frequency Due to
Relative Motion

4. Today,
you
will...
01
02
03
04
Calculate frequency
shifts for various motion
scenarios
Define the Doppler
effect and explain its
basic principles.
Identify the effects of
relative motion on sound
frequency
Explain how Doppler
shift calculations
depend on the medium's
motion.

5. Who introduced Doppler Effect?
Doppler
Effect
The Doppler effect is observed
whenever the source of waves
is moving with respect to an
observer.
What is Doppler Effect?
The Doppler effect is an alteration in the
observed frequency of a sound due to motion of
either the source or the observer.
The effect was proposed (although not fully
worked out) in 1842 by Austrian physicist
Christian Johann Doppler. It was tested
experimentally in 1845 by Buys Ballot in
Holland, “using a locomotive drawing an
open car with several trumpeters.”

6. When the source of the sound (or
other wave) moves toward the
observer, the sound waves are
compressed. This compression
causes the waves to have a shorter
wavelength and a higher frequency,
resulting in a higher-pitched sound.
Conversely, when the source moves
away from the observer, the sound
waves are stretched out, leading to a
longer wavelength and a lower
frequency, producing a lower-pitched
sound.
HOW DOES IT WORKS?

7. BASIC PRINCIPLES OF THE DOPPLER EFFECT
Wave Frequency: The frequency of a wave is the number of cycles it
completes per second.
Relative Motion: The Doppler effect occurs when there is relative motion
between the source of the wave and the observer. This means that either
the source, the observer, or both can be moving.
Wave Speed: The speed of a wave is the distance that the wave travels in
a unit of time. It is determined by the properties of the medium through
which the wave is traveling.
Frequency Shift: When there is relative motion between the source and
the observer, the perceived frequency of the wave changes. If the source
is moving towards the observer, the perceived frequency is higher than
the actual frequency. If the source is moving away from the observer, the
perceived frequency is lower than the actual frequency.

8. Doppler Effect Does not Depend on Distance
Some people think that the Doppler effect depends on the distance between the
source and the observer. Although the intensity of a sound varies as the distance
changes, the apparent frequency depends only on the relative speed of source and
observer. As you listen to an approaching source, you will detect increasing intensity
but constant frequency. As the source passes, you will hear the frequency suddenly
drop to a new constant value and the intensity begin to decrease.
Doppler Effect does depends on the relative
velocity between the source and the observer.

9. Although the Doppler effect is most typically experienced with sound waves,
it is a phenomenon common to all waves. For example, the relative motion of
source and observer produces a frequency shift in light waves. The Doppler
effect is used in police radar systems to measure the speeds of motor
vehicles. Likewise, astronomers use the effect to determine the speeds of
stars, galaxies, and other celestial objects relative to the Earth. In addition,
Doppler shifts can be used to determine velocity, such as when ultrasound is
reflected from blood in a medical diagnostic. In satellite systems, such as
GPS, the Doppler effect helps determine the velocity and position of objects
by analyzing the frequency shifts in signals transmitted from satellites.
APPLICATIONS OF THE DOPPLER EFFECT

11. If either the detector or the source is moving, or both are moving, the emitted
frequency and the detected frequency f’ are related by
speed of sound through the air
GENERAL DOPPLER EQUATION
detector’s speed relative to the air
source’s speed relative to the air
The choice of plus or minus signs is set by this rule:
When the motion of detector or source is toward the other, the sign on its speed must give
an upward shift in frequency. When the motion of detector or source is away from the other,
the sign on its speed must give a downward shift in frequency.

12. No relative motion means the
frequency remains unchanged,
and no Doppler effect is
observed. The sound is heard at
its original pitch.
a. Stationary Source
and Detectors
EFFECTS OF RELATIVE
MOTION ON SOUND
FREQUENCY
Sounds emitted by a source spread out in spherical
waves. Because the source, detectors, and air are
stationary, the wavelength and frequency are the
same in all directions and to all observers.

13. Toward: The sound waves are
compressed, increasing the
frequency, resulting in a higher
pitch.
Away: The sound waves are
stretched, decreasing the
frequency, leading to a lower
pitch.
b. Moving Source and
Stationary Detectors
EFFECTS OF RELATIVE
MOTION ON SOUND
FREQUENCY
Sounds emitted by a source moving to the right
spread out from the points at which they were
emitted. The wavelength is reduced and,
consequently, the frequency is increased in the
direction of motion, so that the detector on the right
hears a higher-pitch sound. The opposite is true for
the detector on the left, where the wavelength is
increased and the frequency is reduced.

14. Toward: The detector
encounters sound waves more
frequently, increasing the
perceived frequency, and the
pitch appears higher.
Away: The detector encounters
sound waves less frequently,
lowering the perceived
frequency, making the pitch
sound lower.
c. Stationary Source
and Moving Detectors
EFFECTS OF RELATIVE
MOTION ON SOUND
FREQUENCY
The same effect is produced when the detectors
move relative to the source. Motion toward the source
increases frequency as the detector on the right
passes through more wave crests than she would if
stationary. Motion away from the source decreases
frequency as the detector on the left passes through
fewer wave crests than he would if stationary.

15. Example Question
An ambulance truck emits a sound with a frequency of
800Hz.
(a) What is the frequency detected by a stationary observer
if the ambulance truck is moving 30 m/s toward the
observer?
(b) What frequency will be detected if the ambulance truck
is moving 30 m/s away from the observer?
(The speed of sound in air at 20C is 343 m/s)

16. Example Question
A stationary ambulance truck emits a frequency of 1200 Hz.
Calculate the frequency detected by the observer if
(a) the observer is driving toward the ambulance truck at 25
m/s and if
(b) the observer is driving away from the ambulance truck at
25 m/s.
(The speed of sound in air at 20C is 343 m/s)

17. Example Question
A police car is moving west at 20 m/s toward a driver who is
moving east at 25 m/s. The police car emits a frequency of
900 Hz.
What frequency is detected by the driver?
(The speed of sound in air at 20C is 343 m/s)

18. Summary
The Doppler Effect is the shift in frequency observed when there is
relative motion between a sound source and an observer. As a
source approaches, the sound waves compress, increasing the
pitch (frequency), while as it moves away, the waves stretch,
lowering the pitch. This phenomenon applies to all waves, including
sound and light, and has practical applications such as in radar
systems, medical diagnostics, and astronomy to measure speed
and distance​
DOPPLER EFFECT

19. Summary
GENERAL EQUATION
Stationary Source and Detectors
Moving Source and Stationary Detectors
Stationary Source and Moving Detectors