DOPPLER EFFECT

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Presentation Transcript

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JP ©

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JP © THE DOPPLER EFFECT

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JP © This is the apparent change in the frequency of a wave motion as noted by an observer when there is relative motion between the source and the observer . THE DOPPLER EFFECT

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JP © The Effect is caused by the Motion of the source, the observer, or both source and the observer. MOVING SOURCE This causes the wave crests to bunch up in front of the source & the wave crests to be more widely spaced behind the source. The overall effect is to cause a change in wavelength. The wave speed is not affected by the motion of the source.

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JP © OBSERVER MOVING THIS CAUSES A CHANGE IN THE APPARENT WAVE SPEED. There is no change in wavelength. If the observer is moving towards the source, the speed of the wavefronts relative to him will be increased and he will receive more waves per second. Moving away from the source the speed of the waves relative to him will be decreased and less waves will pass him per second .

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JP © SOURCE STATIONARY ? CONSTANT, f CONSTANT

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JP © SOURCE MOVING TOWARDS OBSERVER ? - REDUCED, f - INCREASED

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JP © SOURCE MOVING AWAY FROM OBSERVER ? - INCREASED, f - DECREASED

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JP © OBSERVER MOVING TOWARDS SOURCE MEETS MORE WAVES PER SECOND The Effective Velocity of the waves is increased

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JP © OBSERVER MOVING AWAY FROM SOURCE RECEIVES LESS WAVES PER SECOND The Effective Velocity of the waves is reduced

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JP © SOURCE AND OBSERVER MOVING BOTH WAVELENGTH AND EFFECTIVE WAVE VELOCITY ARE CHANGED

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JP © Red shift The colour of light is related to its frequency / wavelength DOPPLER SHIFT IN LIGHT RELATIVE MOTION BETWEEN SOURCE AND OBSERVER CHANGES THE COLOUR OBSERVED. The emission and absorption lines of the light from distant stars and galaxies show a shift towards the red end of the spectrum as compared with those of the same elements in a laboratory source.

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JP © This effect is called red shift . It is interpreted as due to recessional motion of the stars / galaxies from our solar system . This suggests that the Universe is ?? EXPANDING Note that it is possible for some close by stars and galaxies to show a Blue Shift

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JP © Source moving away Red shifted light >>>> Source moving towards Blue shifted light >>>>

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JP © Consider a source emitting light with wavelength ?. The time for one wave to be emitted is the reciprocal of the frequency, i.e. 1/f. Remember that c = f ? , so . If the source is moving away at a speed v, it will have moved away from the observer a distance of ? metres during the time it takes for one wave to be emitted. {distance = velocity x time}

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JP © Hence the change in the wavelength of the light is given by: The fractional change in the wavelength is . This applies as long as v << c .

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JP © The fractional change in frequency is given by a similar formula Identical changes in wavelength and frequency occur if the source is approaching.

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JP © Continuous Spectrum Hydrogen Absorption Spectrum as viewed in Earth Laboratory Hydrogen Absorption Spectrum viewed from a Receding Star ??

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JP © APPLICATIONS 1. Speed of rotation of the Sun 2. Speed of rotation of the Planets 3. Speed of rotation of Saturn’s Rings 4. Speed of rotation of galaxies 5. Speed of recession of galaxies

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JP © What does our friend hear? VELOCITY OF SOURCE = WAVE VELOCITY VELOCITY OF SOURCE > WAVE VELOCITY