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Doppler Effect - Applications
Liu Po Shan Memorial College
Chan Yick Nam
Applications
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Radar Speed Trap
Blood Flow
Red Shift and Blue Shift
Rotating light sources
Radar Speed Trap
Laser Gun
How does it work ?
The police monitor the speeds of vehicles with
laser gun. The laser gun send microwaves
towards the car. The wave reflected back to the
gun have a higher frequency because of the
Doppler effect.
How does it work ?
 The microwave receiver in the laser gun
detects the difference in frequency f
between the emitted signal and the received
signal.

The higher the speed of the car, the greater
the value of f.
f
2v

f
c
Theory
 Path 1:
Waves travel from stationary source to
moving car (observer).
f ’ would be higher than f
Transmitter
Receiver
f
f’
Theory
 Path 2:
Waves reflect back from the moving car
(source) to the receiver (observer).
f ’’ would be higher than f ’ and f
Transmitter
Receiver
f’
f
f”
f’
Theory
 Combining f ’’ and f, beat is formed (f)
f
2v

f
c
 The greater the speed v, the greater the
beat frequency (f) detected
Transmitter
Receiver
f’
f
f”
f’
Limitation
The Laser Gun must be set up with a clear
line-of-sight to the targeted section along
the traffic.
If there are obstacles, such as trees,
telephone poles or road signs, laser
pulses could be reflected/scattered from
these obstacles causing confusions to the
Laser Gun.
Limitation
 It is impossible for a police officer to aim at
vehicles straight ahead, there is an angle
between the line-of-sight of the Laser Gun and
the traveling direction of targeted vehicle.
Limitation
 Thus, the speed measured by the Laser Gun will be less
than the target's “true” speed.
 The larger the angle the lower the measured speed
value. This phenomenon is known as the “Cosine Effect”.
 Therefore, it is important to keep this angle small so that
the speed measured by the Laser Gun is closer to the
true speed.
Blood Flow
Ultrasound
used to
measure blood
flow through
the heart. The
direction of
blood flow is
shown in
different colors
on the screen.
Blood Flow
Ultrasonic waves
reflected from red
blood cells can be
used to find the
velocity of blood flow.
Blood Flow
Narrowing of artery
would increase the
blood flow
Spectrum from Star
Spectrum
A standard lamp
attached inside the
telescope serves as
a comparison
spectrum to allow
the wavelength
shift of the star's
light to be found.
Red Shift and Blue Shift
Red
shift
Blue
shift
 Refer to the direction of the Doppler shift.
 Increase in frequency – shift to blue end (blue shift)
 Decrease in frequency – shift to red end (red shift)
Red Shift and Blue Shift
Shifts in Frequency
Red Shift and Blue Shift
 Approaching sources
Spectral lines shifted to higher frequencies
Shift to short wavelengths. (blue end of visible light
spectrum)
Spectrum is blueshifted
 Receding sources
Lines move to lower frequencies
Shift to longer wavelength (red end of visible light spectrum)
Spectrum is redshifted
Binary Star
Binary Star
Binary Star - Two stars orbiting in a binary system,
held together as a pair by their mutual gravitational
attraction.
Rotating Star
Rotating Star
When the star spins
 One edge approaches, it results a blue shift
 The other edge recedes, it results a red shift.
Overall result: Spectral lines are widened.
The line's width depends on the star's
rotation velocity.
Importance of
Doppler Effect in Astronomy
 The Doppler effect is very important because it
is the only way of measuring the motions of
distant objects.
 To confirm that the universe is expanding.
Links
http://instruct1.cit.cornell.edu/courses/astro101
/lec10.htm
http://resources.emb.gov.hk/physics/articleIE/l
asergun/laserGun__e.htm