1. Doppler Effect - Applications
Liu Po Shan Memorial College
Chan Yick Nam

2. Applications Radar Speed Trap Blood Flow Red Shift and Blue Shift
Rotating light sources

3. Radar Speed Trap

4. Laser Gun

5. 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.

6. 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.

7. Theory
Path 1:
Waves travel from stationary source to moving car (observer).
f ’ would be higher than f f
f ’
Transmitter
Receiver

8. Theory
Path 2:
Waves reflect back from the moving car (source) to the receiver (observer).
f ’’ would be higher than f ’ and f f
f ’
Transmitter
Receiver
f ’ f”

9. Theory Combining f ’’ and f, beat is formed (f)
The greater the speed v, the greater the beat frequency (f) detected f
f ’ f”
Transmitter
Receiver

10. 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.

11. 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.

12. 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.

13. Blood Flow
Ultrasound used to measure blood flow through the heart. The direction of blood flow is shown in different colors on the screen.

14. Blood Flow
Ultrasonic waves reflected from red blood cells can be used to find the velocity of blood flow.

15. Blood Flow
Narrowing of artery would increase the blood flow

16. Spectrum from Star

17. 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.

18. Red Shift and 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)

19. Red Shift and Blue Shift

20. Shifts in Frequency

21. 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

22. Binary Star

23. Binary Star
Binary Star - Two stars orbiting in a binary system, held together as a pair by their mutual gravitational attraction.

24. Rotating Star

25. 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.

26. 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.

27. Links http://instruct1.cit.cornell.edu/courses/astro101/lec10.htm