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Wave properties . The Doppler Effect. B. A. - PowerPoint PPT Presentation


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Wave properties The Doppler Effectis an apparent (observed) change in frequency and wavelength of a wave occurring when the source and observer are in motion relative to each other, with the perceived frequency increasing when the source and observer approach each other and decreasing when they move apart.WHY?Let A and B be two stationary observers.Consider first stationary source (student tapping a desk at a constant pace) : The crests move away from the source at a constant speed. The distance between adjacent crests is one wavelength and is the same toward observer A as toward observer B. The freq of waves reaching both observes is the same and equal to the freq of a wave as it leaves its source.

AB2First wave reaches A and B at the same time. 2nd , 3rd , 4th ,.. wave reaches B sooner than it reaches A. B sees waves coming more frequently i.e. B observes higher frequency and shorter wavelength. Similarly, A observes lower frequency and longer wavelength. General: If the source and the listener are approaching each other the perceived frequency is higher: if they are moving apart, the perceived frequency is lower.

ABNow: source moves to the right at speed < wave speed.Each new wave originates from the point farther to the right.34If a source of sound is moving toward you at constant speed, you hear a higher freq than when it is at restIf it is moving at increasing speed you hear higher and higher freqIf a source of sound is moving away from you, you hear a lower freq than when it is at restIf it is moving at increasing speed you hear lower and lower freqYou can hear this effect with sirens on fire engines of train whistlesA similar effect occurs with light waves and radar waves


Ultrasound (high-frequency sound waves) are directed into an artery. The waves are reflected by blood cells back to a receiver. The frequency detected at the receiver fr relative to that emitted by the source f indicates the cells speed and the speed of the blood.ApplicationsA similar arrangement is used to measure the speed of cars, but microwaves (EM waves) are used instead of ultrasound.

The Doppler effect is the basis of a technique used to measure the speed of flow of blood.6Doppler effect Radar guns

http://auto.howstuffworks.com/radar-detector1.htmWhen radar waves bounce off a moving object(echo ) the frequency of the reflected wave changesby an amount that depends on how fast the objectis moving. The detector senses the frequencyshift and translates this into a speed.

7Light or in general EM wave is a wave. Doppler effect is the characteristic of these waves too.

Based on calculations using the Doppler effect, it appears that nearby galaxies are moving away from us at speed of about 250,000 m/s. The distant galaxies are moving away at speeds up to 90 percent the speed of light. The universe is moving apart and expanding in all directions. 8

Freqs. of EM waves coming from stars are very often lower than those obtained in the laboratory emitted from same elements (He, H). Redshift shift toward lower freq. Red light has the lowest frequency out of all of the visible lights. Astronomy: the velocities of distant galaxies can be determined from the Doppler shift.Most distant galaxies are observed to be red-shifted in the color of their light, which indicates that they are moving away from the Earth. Some galaxies, however, are moving toward us, and their light shows a blue shift.Edwin Hubble discovered the Redshift in the 1920's. His discovery led to him formulating the Big Bang Theory of the Universe's origin.9

A science teacherdemonstrating theDoppler effect10When a wave strikes a boundary between two media some of it is reflected, some is absorbed and some of it is transmitted. We now look to see what happens to a wave when it is incident on the boundary between two media.How much of each?That depends on the media and the wave itself.

Reflection of Waves All waves can be reflected.


The end of the rope if fixed reflected pulse returns inverted. Imagine whip

Some of the energy of the pulse will actually be absorbed at the support and as such, the amplitude of the reflected pulse will be less than that of the incident pulse (E ~ A2).

the pulse has undergone a 180 () phase change.there is no phase change. First of all, we shall look at a single pulse travelling along a string.Free end reflected pulse is not inverted. fixed end


Law of ReflectionThe incident and reflected wavefronts.Angle of reflection is equal to angle of incidence. All waves, including light, sound, water obey this relationship, the law of reflection. i = r .(the angles are measured to the normal to the barrier).

RefractionWhen a wave passes from one medium to another, its velocity changes. The change in speed results in a change in direction of propagation of the refracted wave.

As a toy car rolls from a hardwood floor onto carpet, it changes direction because the wheel that hits the carpet first is slowed down first. Visualization of refraction

When a wave passes from one medium to another, its velocity changes. The change in speed results in a change in direction of propagation of the refracted wave.

light wavessound wavesThe incident and refracted wavefronts.frequency is determined by the source so it doesnt change. Only wavelenght changes. Wavelength of the same wave is smaller in the medium with smaller speed.


2A mathematical law which will tell us exactly HOW MUCH the direction has changed is called SNELL'S LAW.

Although it can be derived by using little geometry and algebra, it was introduced as experimental law for light in 1621.

For a given pair of media, the ratiois constant for the given frequency. The Snells law is of course valid for all types of waves.18

Refraction occurs when a wave enters a medium with different speed of wave.When the wave enters a medium with slower speed the wavelength becomes shorter and the wave speed decreases. The frequency remains the same. www.le.ac.uk/ua/mjm33/wave2/images/Snell.gif

The speed of light inside matterThe speed of light c = 300,000,000 m/s = 3 x 108 m/sIn any other medium such as water or glass, light travels at a lower speed.

INDEX OF REFRACTION, n, of the medium is the ratio of the speed of light in a vacuum, c, and the speed of light, v, in that medium:

no units As c is greater than v for all media, n will always be > 1. greater n smaller speed of light in the medium. As the speed of light in air is almost equal to c, nair ~ 1

MEDIUMnv (m/s)VacuumairExactly 11.000293300,000,000water1.33225,564,000glass1.52197,368,000diamond2.42123,967,000Refraction of lightWater n= 1.33Glass (n=1.5)IncidentrayrefractedrayThe refracted ray is refracted more in the glass22SNELL'S LAW

Can be written in another form for refraction of light only.greater n smaller speed of light stronger refraction smaller angle

Which of the three drawings (if any) show physically possible refraction?

Answer: (a) refraction toward normal as it should be24

White lightcontains allwavelengths(colors)

RedOrangeYellowGreenBlueIndigoVioletDispersionEven though all colors of the visible spectrum travel with the same speed in vacuum, the speed of the colors of the visible spectrum varies when they pass through a transparent medium like glass and water. That is, the refractive index of glass is different for different colours.Different colors are refracted by different amounts.

qcq2=900n2n1 Total internal reflection

Angle of refraction is greater than angle of incidence. As the angle of incidence increases, so does angle of refraction. The intensity of refracted light decreases, intensity of reflected light increases until angle of incidence is such that angle of refraction is 900.Critical angle: qc - angle of incidence for which angle of refraction is 900 When the incident angle is greater than qc , the refracted ray disappearsand the incident ray is totally reflected back.26Critical angle: qc - angle of incidence for which angle of refraction is 900

A Smile in the Sky1. When white sunlight enters droplet its component colors are refracted at different angles (dispersion)2. These colored lights then undergo total internal reflection.Rainbows are caused by dispersion of sunlight by water droplets3. Second refraction from droplet into air more dispersion4. Each droplet produces a complete spectrum, but only one from each is seen by observer you have your own personal rainbow and I have MINE!observer is between the Sun and a rain shower.


A gemstone's brilliance is caused by total internal reflectionA gemstone's "fire" is caused by dispersion

Maximize brilliance & fire by knowing physicsWhat does it mean to see something?To see something, light rays from the object must get into your eyes.unless the object if a light bulb or some other luminous object, the light rays from some light source (like the sun) reflect off of the object and enter our eyes.

(root)BEER!30Where is the fish? Deeper than you think!Apparent location of the fish


even the long legs are pretty short in the water32Is the straw really broken?refracted ray

real strawincident rayperceived straw

33Where is the ball? Closer than you think!ballApparent location of the ball


It looks like a moon setting over a body of water, with the moon's reflection in the surface. We can even see floating lea