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# A stationary source of wave emissions propagates those waves into spac

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Retired Moderator
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A stationary source of wave emissions propagates those waves into spac  [#permalink]

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Updated on: 06 Oct 2019, 22:54
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A stationary source of wave emissions propagates those waves into space at a constant given frequency which will be perceived by a stationary observer at the frequency at which the waves were emitted, taking into consideration the effect of the medium through which the waves travel. An object, moving with respect to an observer at a velocity less than that of the wave it emits, creates a change known as the Doppler Effect in its perceived output. The waves emitted from this source continue their expansion at a constant rate from the source’s original location so that the center of each wavefront is shifted according to direction and velocity. Should the observer and source be approaching each other’s position, the wavelengths become shorter and the frequency becomes higher with a subsequent apparent sensory effect, the reverse being true should the two be moving away from each other. The total Doppler Effect, therefore, may be used to detect the velocity and direction of any object which emits waves, if enough information is available about the state of the observer and the medium through which the waves are propagated. For instance, the electromagnetic radiation emitted by objects in space outside our galaxy exhibits a definite redshift, those objects farther away from us being more redshifted than those closer to us, whereas objects within our galaxy may exhibit blue or redshifts. Edwin Hubble used these facts to argue that the universe is constantly expanding, carefully noting that taking the same readings from any location in the universe would result in the same findings.

Which of the following is suggested about redshifts?

A redshift shows that an object which emits waves is moving away from an observer.
A redshift is an indicator of the distance between an object and its observer.
A redshift is the result of a higher frequency of waves than that which the source originally emitted reaching the observer.
A redshift occurs when the distance between a moving source of waves and a stationary observer increases.
An object is more redshifted the greater the distance between that object and an observer.

Answer Choice A; We know that we can learn about an object's velocity and direction from the Doppler Effect, which produces apparent sensory effects, in this case a light which appears more red from the observer's perspective than it normally is, i.e. a redshift. Since Hubble used these redshifts to argue that the universe is constantly expanding, we may infer that if an object indicates a redshift (as opposed to a blueshift) it is moving away from the observer. We may also infer that the more redshifted an object is, the faster away it is moving from the observer.

Originally posted by reto on 04 Sep 2015, 10:36.
Last edited by SajjadAhmad on 06 Oct 2019, 22:54, edited 2 times in total.
Updated - Complete topic (808).
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Re: A stationary source of wave emissions propagates those waves into spac  [#permalink]

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14 Sep 2015, 03:04
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reto wrote:
What is your technique with long passages? You need to have a system, it is more important to understand the scope, tone, organization and purpose of the passage in order to answer the questions.

Primarily, This is a short passage.
Secondly, My technique is I do read the passage first and then answer the questions.
but unfortunately, I could not understand this passage completely.
I understood that the more the object is distant the more redshift is generated.
But I could not differentiate the options since almost all sounded correct/wrong to me.
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Re: A stationary source of wave emissions propagates those waves into spac  [#permalink]

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31 May 2018, 04:01
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It may be inferred that a moving observer could perceive the waves emitted by a moving object
A. at a velocity other than their real velocity
B. unless the object is traveling faster than the waves it emits
C. unless the medium through which the waves travel interferes
D. as if the object were stationary
E. unless the two are traveling away from each other at a speed different than the speed of the waves emitted by the object

I feel both A and D are correct . The question asks which COULD be true.
A: When the velocity of moving object and that of observer are different , the observer could perceive a different velocity .
D.When the velocity of moving object and that of observer are same and oth moving object and observer travel in the same direction , then perceived velocity will not change .

So , I a sure A and D both are right
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Re: A stationary source of wave emissions propagates those waves into spac  [#permalink]

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20 Jul 2018, 07:51
pkm9995109794 wrote:
E cannot be wrong in that aspect.

E is wrong. Because the Doppler effect happens when source move. Choice E is just talking about distance.
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Re: A stationary source of wave emissions propagates those waves into spac  [#permalink]

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23 Nov 2019, 08:19
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Re: A stationary source of wave emissions propagates those waves into spac   [#permalink] 23 Nov 2019, 08:19
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