Practical Applications Of Doppler Effect For Light

2018-01-26 04:11:20

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In the science and technology driven life-style, in sciences, we generally teach concepts without bothering to highlight their practical applications in science and technology. Many of these science based technologies might have been taken into our daily life use and we use them without bothering to go into the basics of these technologies ourselves or without making the students to correlate the use of scientific concepts governing these technological applications. One of such science concept is relativistic Doppler Effect of light which is a direct consequence of Einstein’s special theory of relativity.  The Doppler effect is used in many technologies that benefit people. Thinking that the basic concept itself is beyond the grasp/understanding of general class of students and sometimes difficult for teachers also to go into the elaboration of various growing technological applications based on this concept, this article presents a number of advanced and ordinary applications based on the Doppler Effect of light to highlight their day to day use.

Relativistic Doppler Effect for Light

Doppler effect has been discussed generally in terms of sound waves but Doppler himself maintained that it could be applied to light waves as well, and experimentation conducted in 1901 proved him correct. This was far from an obvious point, since light is quite different from sound. Not only does light travel much, much faster but unlike sound, light does not need to travel through a medium. Whereas sound cannot be transmitted in outer space, light is transmitted by radiation, a form of energy transfer that can be directed as easily through a vacuum as through matter. Consider two objects, the light source and the listener (or observer). Since light waves traveling in empty space (no medium), we analyze the Doppler Effect for light in terms of the motion of the source relative to the listener. We set up our coordinate system so that the positive direction is from the listener toward the source. So if the source is moving away from the listener, its velocity  is positive, but if it is moving toward the listener, then the velocity is negative. The listener, in this case, is always considered to be at rest (so velocity is really the total relative velocity between them). The speed of light is always considered positive. The listener receives a frequency which would be different from the frequency transmitted by the source. This is calculated with relativistic mechanics, by applying necessary the length contraction. A light source moving away from the listener (velocity is positive) would provide a frequency that is less than the frequency of source. In the visible light spectrum, this causes a shift toward the red end of the light spectrum, so it is called a red shift. When the light source is moving toward the listener (velocity is negative), then frequency of listener is greater than frequency of source. In the visible light spectrum, this causes a shift toward the high-frequency end of the light spectrum and such frequency shift is actually called a blue shift. Obviously, in the area of the electromagnetic spectrum outside of the visible light spectrum, these shifts might not actually be toward red and blue. If you're in the infrared, for example, you're ironically shifting away from red when you experience a "red shift."

Demonstration of Doppler Effect of Light

The Doppler Effect in light can be demonstrated by using a device called a spectroscope, which measures the spectral lines from an object of known chemical composition. These spectral lines are produced either by the absorption or emission of specific frequencies of light by electrons in the source material. If the light waves appear at the blue, or high-frequency end of the visible light spectrum, this means that the object is moving toward the observer. If, on the other hand, the light waves appear at the red or low-frequency end of the spectrum, the object is moving away. Today, scientists know that the Doppler Effect applies to all types of waves, including water, sound and light. They also have a good idea why the Doppler effect occurs. And they've incorporated its principles into a variety of useful tools and gadgets.

Applications of Doppler Effect of Light

In the 160 years or so since Doppler first described the wave phenomenon that would cement his place in history, several practical applications of the Doppler effect have emerged to serve society. In all of these applications, the same basic thing is happening: a stationary transmitter shoots waves at a moving object. The waves hit the object and bounce back. The transmitter (now a receiver) detects the frequency of the returned waves. Based on the amount of the Doppler shift, the speed of the object can be determined. Some of these applications are mentioned as:

1.      Expanding universe

In 1923, American astronomer Edwin Hubble (1889-1953) observed that the light waves from distant galaxies were shifted so much to the red end of the light spectrum that they must be moving away from the Milky Way, the galaxy in which Earth is located, at a high rate. At the same time, nearer galaxies experienced much less of a red shift, as this phenomenon came to be known, meaning that they were moving away at relatively slower speeds. Six years later, Hubble and another astronomer, Milton Humason, developed a mathematical formula whereby astronomers could determine the distance to another galaxy by measuring that galaxy's red shifts. The formula came to be known as Hubble's constant, and it established the relationship between red shift and the velocity at which a galaxy or object was receding from Earth. From Hubble's work, it became clear that the universe was expanding, and research by a number of physicists and astronomers led to the development of the "big bang" theory—the idea that the universe emerged almost instantaneously, in some sort of explosion, from a compressed state of matter.

2.      Use in Radar

The Doppler effect is used in some types of radar, to measure the velocity of detected objects. A radar beam is fired at a moving target — e.g. a motor car, as police use radar to detect speeding motorists — as it approaches or recedes from the radar source. Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gap between each wave increases, increasing the wavelength. In some situations, the radar beam is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength. In either situation, calculations from the Doppler effect accurately determine the car's velocity. Moreover, the proximity fuze, developed during World War II, relies upon Doppler radar to detonate explosives at the correct time, height, distance, etc. Because the doppler shift affects the wave incident upon the target as well as the wave reflected back to the radar, the change in frequency observed by a radar due to a target moving at relative velocity.

2.1  Tracking a satellite

Fast moving satellites can have a Doppler shift of dozens of kilohertz relative to a ground station. The speed, thus magnitude of Doppler effect, changes due to earth curvature. Dynamic Doppler compensation, where the frequency of a signal is changed multiple times during transmission, is used so the satellite receives a constant frequency signal. The Doppler Effect provides a convenient means of tracking a satellite that is emitting a radio signal of constant frequency. The frequency of the signal received on the Earth changes as the satellite is passing. If the received signal is combined with a constant signal generated in the receiver to give rise to beats, then the beat can have a frequency that produces an audible note, whose pitch decreases as the satellite passes overhead. By observing how the frequency changes, we can determine the velocity relative to our location, which allows ground-based tracking to analyze the movement of objects in space.

2.2  Tool for measuring speed

The handheld radar guns used by police to check for speeding vehicles rely on the Doppler Effect. Here's how they work: A police officer takes a position on the side of the road. The officer aims his radar gun at an approaching vehicle. The gun sends out a burst of radio waves at a particular frequency. The radio waves strike the vehicle and bounce back toward the radar gun.The radar gun measures the frequency of the returning waves. Because the car is moving toward the gun, the frequency of the returning waves will be higher than the frequency of the waves initially transmitted by the gun. The faster the car's speed, the higher the frequency of the returning wave. The difference between the emitted frequency and the reflected frequency is used to determine the speed of the vehicle. A computer inside the gun performs the calculation instantly and displays a speed to the officer.

2.2.1 Speed guns used in tennis and circlet

A speed-gun is placed right next to the sight screen. It emits microwaves to analyze the trajectory of the ball through the air, using which the velocity of the ball can be calculated at all times. Microwaves are used because they have a large wavelength and hence they can pierce through the air with lesser deviation as compared to waves having lower wavelengths. The data obtained using the radar gun is fed to image processing software which identifies the ball among the other objects on the pitch and tells the speed of the ball. As the microwaves have a very small wavelength thus the reading is not affected by the wind conditions. Now, since the velocity of the ball keeps varying, so the time taken for the ball to travel a fixed distance is kept as standard for the calculation of speeds. Since weather conditions, air-speeds, etc keep varying from venue to venue, average bowling speeds tend to vary slightly everywhere, usually.

  • Exact speed is determined with help of a radar gun as it catches the speed of the moving ball the way it is without any error.
  • It is instantaneous and records the speed immediately as the ball passes the radar gun. This is the reason that in any cricket match, as soon as the bowler balls the delivery, the speed is shown on the screen.
  • The gun works efficiently and helps getting the exact possible speed of the bowl.

2.3  Weather Doppler Radar

Meteorologists use a similar principle to read weather events. In this case, the stationary transmitter is located in a weather station and the moving object being studied is a storm system. This is what happens: Radio waves are emitted from a weather station at a specific frequency. The waves are large enough to interact with clouds and other atmospheric objects. The waves strike objects and bounce back toward the station. If the clouds or precipitation are moving away from the station, the frequency of the waves reflected back decreases. If the clouds or precipitation are moving toward the station, the frequency of the waves reflected back increases. Computers in the radar electronically convert Doppler shift data about the reflected radi waves into pictures showing wind speeds and direction. Doppler images are not the same as reflectivity images. Reflectivity images also rely on radar, but they are not based on changes in wave frequency. Instead, a weather station sends out a beam of energy, then measures how much of that beam is reflected back. This data is used to form the precipitation intensity images we see all the time on weather maps, where blue is light precipitation and red is heavy precipitation.

3.      Medical imaging

There are also a number of medical applications of the Doppler effect found in ultrasonography, echocardiography, and radiology, all of which employ ultrasonic waves.  Colour flow ultrasonography (Doppler) of a carotid artery - scanner and screen. An echocardiogram can, within certain limits, produce an accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the ultrasound beam should be as parallel to the blood flow as possible. Velocity measurements allow assessment of cardiac valve areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves (valvular regurgitation), and calculation of the cardiac output. Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements. Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (when the received signal arrives). Velocity measurements of blood flow are also used in other fields of medical ultrasonography, such as obstetric ultrasonographyand neurology. Velocity measurement of blood flow in arteries and veins based on Doppler effect is an effective tool for diagnosis of vascular problems like stenosis.

4.      Flow measurement

Instruments such as the laser Doppler velocimeter (LDV), and acoustic Doppler velocimeter (ADV) have been developed to measure velocities in a fluid flow. The LDV emits a light beam and the ADV emits an ultrasonic acoustic burst, and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow is computed as a function of the water velocity and phase. This technique allows non-intrusive flow measurements, at high precision and high frequency. Developed originally for velocity measurements in medical applications (blood flow), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions. Flows can be pulsating, oscillating, laminar or turbulent, stationary or transient. This technique is fully non-invasive.

5.      Doppler effect-based fiber-optic sensor and its application in ultrasonic detection.

Based on the Doppler effect of light wave transmission in optical fiber, Doppler effect-based fiber-optic (FOD) sensor possesses outstanding advantages in acquiring vibration/acoustic waves with high sensitivity. Furthermore, when shape of the FOD sensor was properly selected, its sensitivity was bonding direction-independent, namely non-directionality. Features of the ultrasonic wave signals, collected using a number of spiral FOD sensors with various inner diameters and outer diameters, can be compared to investigate characteristics of FOD sensor. Amplitude curves of the FOD sensors are obtained for the applications in ultrasonic acquisition.

Busting the Boom

Doppler effect can have a negative impact, as well, for example, sonic booms, which are caused by supersonic aircraft, can cause objectionable sounds and vibrations on the ground, which is why supersonic airplanes are not allowed to fly over populated areas. Sonic booms are directly related to the Doppler Effect. They occur when airplanes, flying at the speed of sound or higher, actually fly faster than the sound waves they are producing. All of the waves bunch up behind the craft, in an extremely small space. When the bunched-up waves reach an observer, they are "heard" all at once -- as a resounding boom. Scientists and engineers are experimenting with several inventions that help mitigate sonic booms. One such invention is a spike extending from the nose of the airplane. This spike essentially lengthens the plane and distributes the waves over a greater distance. This reduces the boom experienced by an observer on the ground.