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.

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.

How does it work ?

c

v2

f

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 ’

f ’f”

Transmitter

Receiver

Theory

Combining f ’’ and f, beat is formed (f)

f f ’

f ’f”

Transmitter

Receiver

c

v2

f

f

The greater the speed v, the greater the beat frequency (f) detected

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.

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

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

Redshift

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 spectru

m) Spectrum is blueshifted

Receding sources Lines move to lower frequencies Shift to longer wavelength (red end of visible light spectru

m) 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 rot

ation 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