by zermeen fatima

1. Doppler EffectDoppler Effect

3. HISTORY
• The Doppler effect was
named after Christian
Doppler, who first came up
with the idea in 1842.
• The Doppler effect
doesn't just apply to
sound. It works with all
types of waves, which
includes light. Edwin
Hubble used the Doppler
effect to determine that
the universe is expanding.

4. As a wave source approaches, an observer
encounters waves with a higher frequency. As the
wave source moves away, an observer encounters
waves with a lower frequency. This apparent
change in frequency due to the motion of the
source (or receiver) is called the Doppler effect.
Definition

5. The greater the speed of the source, the greater will be the
Doppler effect.

6. Doppler Effect In:
• SOUND
• LIGHT

7. Sound
The Doppler effect causes the changing pitch of a siren.
When a fire truck approaches, the pitch sounds higher than
normal because the sound wave crests arrive more frequently.
When the fire truck passes and moves away, you hear a drop
in pitch because the wave crests are arriving less frequently.
The change in loudness is not the Doppler
Effect! It is the shift in frequency!

8. • 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.

9. Light
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.

10. Astronomy and the Doppler
Shift
• Astronomers can measure the
Doppler Shift of a moving
object.
• The radio wave has a known
frequency for stationary
objects.
• If target is moving away,
waves in the signal will be
stretched, and will have a
lower frequency.
• If the target is moving closer,
waves will be compressed and
will have a higher frequency.

11. • Increasing frequency is called a blue shift , because the increase
is towards 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.

12. QUESTION

13. • When a source moves towards you, do
you measure an increase or decrease
in wave speed?

14. Neither! It is the frequency of a wave
that undergoes a change, not the wave
speed.

15. CasesCases

37. Case 7
• If the source of sound is moved away from the
observer at rest towards wall, then in this case
observer hears two nodes of different
frequencies.
• The note heard directly from the receding source
is lowered in pitch by the motion.
• The other note is due to the waves reflected
from the wall, which is raised in pitch , that is so
because the source is moving towards the wall.
• The superposition of these two waves produce
beats.

38. APPLICATIONSAPPLICATIONS

39. WEATHER STATIONS
• Radio waves are emitted from a
weather station at a specific
frequency.
• The waves are large enough to
interact with clouds and other
atmospheric objects.
• The waves strike the objects and
bounce back towards the Station.
• If the clouds or precipitation are
moving away from the Station,
the frequency of the waves
reflected back decrease.
• If the clouds or precipitation are
moving towards the station, the
frequency of the waves reflected
back increase.

40. POLICE RADAR
SYSTEMS
A police officer aims his radar gun at an approaching
vehicle. The gun sends out a burst of radio waves at a
particular frequency.
The radio waves strike the vehicle and bounce back toward
the radar gun.
An electromagnetic wave is emitted by a source at the side
of the road attached to a Police Car. The Wave is reflected
by a moving vehicle, which thus acts as a moving source.
The reflected wave will have a Doppler Shift in frequency.
This frequency shift is measured by using the phenomenon
of beats, which permits the measurement of the speed of
vehicle.

41. BATS
• A bat determines the
location and nature of
objects by sending
ultrasonic waves.
• The bats are provided
with some sensory
devices to find speed of
the objects in their
surrounding by sensing
the Doppler frequency
shift.

42. AEROPLANES
• Doppler Shift is also
applicable in radar
system. The radar is a
device which
transmits and receives
radio waves from the
aeroplanes if an
aeroplane is coming.

43. ASTRONOMY
• The relativistic Doppler
effect is important in
astronomy, if the source is
receding at high speed from
an observer, then a lower
frequency (or longer
wavelength) is observed for
certain spectral or
characteristic frequency.
• This is the origin of the
term Redshift, the
wavelength of visible light
are shifted towards longer
wavelength (red).

44. ANY QUESTIONS???