7.12 Applications of Doppler Effect

7.12 Applications of Doppler Effect

Doppler effect is also applicable to electromagnetic waves. One of its important applications is the radar system, which uses radio waves to determine the elevation and speed of an aeroplane. RADAR (Radio Detection And Ranging) is a device, which transmits and receives radio waves. If an aeroplane approaches towards the radar, then the wavelength of the wave reflected from aeroplane would be shorter and if it moves away, then the wavelength would be larger as shown in Fig. 7.15 (a) & Fig. 7.15 (b), respectively. Similarly, speed of satellites moving around the Earth can also be determined by the same principle.

SONAR is an acronym derived from "Sound Navigation And Ranging". It is the general name for sonic or ultrasonic underwater echo-ranging and echo-sounding system. Sonar is the name of a technique for detecting the presence of objects under water by acoustical echo. In Sonar, "Doppler detection" relies upon the relative speed of the target and the detector to provide an indication of the target speed. It employs the Doppler effect, in which an apparent change in frequency occurs when the source and the observer are in relative motion to one another. Its known military applications include the detection and location of submarines, control of antisubmarine weapons, mine hunting and depth measurement of sea.

In Astronomy, astronomers use the Doppler effect to calculate the speeds of distant stars and galaxies. By comparing the line spectrum of light from the star with light from a laboratory source, the Doppler shift of the star's light can be measured. Then, the speed of the star can be calculated.

(i) Stars moving away from the Earth show a red shift as shown in Fig. 7.16(b). The emitted waves have a longer wavelength than if the star had been at rest. So, the spectrum is shifted towards longer wavelength, i.e., towards the red end of the spectrum. Astronomers have also discovered that all the distant galaxies are moving away from us and by measuring their red shifts, they have estimated their speeds.

(ii) Stars moving towards the Earth show a blue shift as shown in Fig. 7.16(c). This is because the wavelength of light emitted by the star are shorter than if the star had been at rest. So, the spectrum is shifted towards shorter wavelength, i.e., to the blue end of the spectrum.

Another important application of the Doppler shift using electromagnetic waves is the radar speed trap. Microwaves are emitted from a transmitter in short bursts. Each burst is reflected off by any car in the path of microwaves in between sending out bursts. The transmitter is open to detect reflected microwaves. If the reflection is caused by a moving obstacle, the reflected microwaves are Doppler shifted. By measuring the Doppler shift, the speed at which the car moves is calculated by computer programme.

Satellite Navigation uses Doppler shift to determine satellite velocity and position, enabling accurate location tracking.

Satellite Communication also uses Doppler shift compensation ensuring stable communication signals.

Doppler radar detects wind velocity and precipitation patterns. Doppler shift helps measure Earth's surface velocity and deformation.

Doppler echocardiography measures blood flow velocity and detects cardiac abnormalities, such as valve stenosis or regurgitation. Doppler echocardiography optimizes pacemaker settings.

Doppler ultrasound measures blood flow and calculates cardiac output. Doppler ultrasound detects vascular stenosis or occlusion.

Example 7.12

The wavelength of one of its lines of the absorption spectrum of a faint galaxy is identified as $\mathrm{Ca}\text{-}\alpha$ line found to be $478\,\mathrm{nm}$. The wavelength of same line is observed and measured as $397\,\mathrm{nm}$ in the laboratory.

(a) Is the galaxy moving towards or away from the Earth? (b) Compute the speed of the galaxy relative to the Earth.

Solution

Laboratory measured original wavelength $\lambda = 397\,\mathrm{nm} = 397 \times 10^{-9}\,\mathrm{m}$

Changed or Apparent wavelength $\lambda' = 478\,\mathrm{nm} = 478 \times 10^{-9}\,\mathrm{m}$

Speed of light $c = 3 \times 10^{8}\,\mathrm{m\,s^{-1}}$

(a) As $\lambda' > \lambda$, the galaxy is moving away from the Earth.

(b) Laboratory frequency:

Apparent frequency:

Putting the values, we have: