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

Applications 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.

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

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”

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

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 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/lasergun/laserGun__e.htm