# 5.6 The doppler effect  (Page 2/35)

 Page 2 / 35

The situation is not the same for other observers. Let’s look at the situation from the point of view of observer C , located opposite observer A in the figure. For her, the source is moving away from her location. As a result, the waves are not squeezed together but instead are spread out by the motion of the source. The crests arrive with an increased wavelength and decreased frequency. To observer B , in a direction at right angles to the motion of the source, no effect is observed. The wavelength and frequency remain the same as they were in part (a) of the figure.

We can see from this illustration that the Doppler effect is produced only by a motion toward or away from the observer, a motion called radial velocity    . Sideways motion does not produce such an effect. Observers between A and B would observe some shortening of the light waves for that part of the motion of the source that is along their line of sight. Observers between B and C would observe lengthening of the light waves that are along their line of sight.

You may have heard the Doppler effect with sound waves. When a train whistle or police siren approaches you and then moves away, you will notice a decrease in the pitch (which is how human senses interpret sound wave frequency) of the sound waves. Compared to the waves at rest, they have changed from slightly more frequent when coming toward you, to slightly less frequent when moving away from you.

## Color shifts

When the source of waves moves toward you, the wavelength decreases a bit. If the waves involved are visible light, then the colors of the light change slightly. As wavelength decreases, they shift toward the blue end of the spectrum: astronomers call this a blueshift (since the end of the spectrum is really violet, the term should probably be violetshift , but blue is a more common color). When the source moves away from you and the wavelength gets longer, we call the change in colors a redshift . Because the Doppler effect was first used with visible light in astronomy, the terms “ blueshift ” and “ redshift ” became well established. Today, astronomers use these words to describe changes in the wavelengths of radio waves or X-rays as comfortably as they use them to describe changes in visible light.

The greater the motion toward or away from us, the greater the Doppler shift. If the relative motion is entirely along the line of sight, the formula for the Doppler shift of light is

$\frac{\text{Δ}\text{λ}}{\text{λ}}=\frac{v}{c}$

where λ is the wavelength emitted by the source, Δλ is the difference between λ and the wavelength measured by the observer, c is the speed of light, and v is the relative speed of the observer and the source in the line of sight. The variable v is counted as positive if the velocity is one of recession, and negative if it is one of approach. Solving this equation for the velocity, we find v = c × Δλ/λ.

#### Questions & Answers

what is the model about the earth being the center of the universe called?
Lordgy Reply
geocentric model
SCIENCE
I hope you got your answer.
SCIENCE
heliocentric universe
Jason
you are wrong Jason skerbinc
SCIENCE
in heliocentric model sun is at the center of the universe
Tanmay
what is black hole and stellar black hole
azeb Reply
does Millikan consider electron axial spin in his discovery of electrons mass using q/m ratio ? If No how it is possible that he found rest mass of electron (9×10^31 Kg) ?
Anurag Reply
how did light from up
Sourav Reply
Before there stars , there was matter and radiation . Before there were neutral atoms ,there was an ionized plasma ,and when that plasma froms neutral atoms , those allow the Universe to deliver the earliest light we see today .......
Apsara
how to know the distance of a star
Sourav Reply
What is time
Sourav
Technical Infinite Measurement of Evolution
Doc
not by the book but the simplicity of word play ....
Doc
to calculate the distance of a star we use parallax method (as far as my school has taught me).
Tanmay
in this method we see the star from one point and then after traveling some distance perpendicular to the distance of star we see it again. With this we can make a triangle and then it's median will give our distance
Tanmay
And time is just a way of defining when an event happened
Tanmay
oh and we will know the distance of all the 3 sides of triangle formed above if we multiply the time taken by light with speed of light and the third side will be known to us as it is dependent on our will
Tanmay
what are wormholes?
facty Reply
Holes made by worms
Neil
What is the difference between black hole and white hole ?
Apsara
black hole is the place where the massive body makes a large disturbance in space time fabric causing large gravity even light can't escape from it and all the bodies enter into on reaching a specific distance.
SCIENCE
A white hole is a hypothetical region of spacetime and singularity which cannot be entered from the outside, although energy-matter and light can escape from it. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter and light can
SCIENCE
White holes appear in the theory of eternal black holes
SCIENCE
worm holes join 2 distant region of spacetime through which you can travel, covering large distance in short time
Tanmay
for example if you need to travel from A to B on a plane of paper then you can travel in a straight line joining those points but a worm hole will join those points using 3rd dimention which will make you travel that distance in short time
Tanmay
Hi,Do you known Kyaw Nyein Chan Hein!
zin Reply
No
Astrophysicist
Hello : I'm Kyaw Nyein Chan
zin
Hi I'm Kyaw Nyein Chan
zin Reply
Hello : good moring, I'm Kyaw Nyein Chan
zin Reply
hello good morning
Astrophysicist
good morning bro
Tanmay
how do you do?
Astrophysicist
I am fine what about you?
Tanmay
same as you
Astrophysicist
cool
Tanmay
How the Big Bang is made .
Apsara Reply
Kyaw Nyein Chan
zin
What is heliocentric
Apsara Reply
helio is from the greek god helios or ilios and means sun. Centric is from central. So heliocentric means with the sun in the middle
Robin
Usually in the context of astronomy its about the moment the world goes from the idea of 'earth in the middle of the solar system' to the idea that the sun is in the center of the solar system
Robin
so from a geocentric (geo meaning earth) to a heliocentric idea
Robin
yes correct this theory states that sun is in the middle of the solar system rather than earth
Tanmay
oh you have already said this
Tanmay
Hello: good afternoon
zin
hello
Tanmay
Thankyou to all to solve my doubt . I don't know more about the solar system beacause i am a small little girl my old is 9 and i love science but i am trying to learn more about science .
Apsara
that's great you should definitely check science in a nutshell YouTube channel it will provide you with good information
Tanmay
that's great you should definitely check science in a nutshell YouTube channel it will provide you with good information
Tanmay
that's great you should definitely check science in a nutshell YouTube channel it will provide you with good information
Tanmay
here is the link ***youtube.com/c/inanutshell
Tanmay
You not alone Apsara I'm 14 but know little about the universe
Swazi
what is solar system
Himanshi
what do you mean by astronomy
Himanshi
astronomy is a study of planetary science.
Enceladus
Astronomy is a branch of Physics that deals with the study of celestial bodies.
Satyam
An astronomy is a scientist in the field of astronomy who focuses their studies on a specific questions or field outside the scope of Earth 🌍🌎🌏 . They observe astronomical objects such as star's , planets , moon , comets and galaxies - in either observational (by analyzing the data)
Apsara
what is the difference between astronomy and cosmology
Astrophysicist
The collection of eight planets and their moon's in orbit around the Sun , together with smaller bodies in the form of asteroids , meteoroids and comets . The planet of the solar system are ( in order of distance from the sun ) Mercury , Venues , Earth ,Mass , Jupiter , Sartune ,Uranus , Neptune .
Apsara
In Astronomy, we study about celestial objects but in cosmology we study the origin and the formation of universe.
Satyam
how much temperature were can find on neptune
Ayyan Reply
the temperature is between -200 to -218 degree Celsius
Tanmay
how can I apply online astronomy course ? if you have link send it.
Dawit Reply
what is astronomy
Khanyisa
Does any body know about Quantum Battery ?
Yabtsega Reply
yea I tell you briefly callme
Eyiba
How to get into the field of astronomy after completing B.tech in department of ECE?
Lakshmi Reply
Well my option is to do your job while getting a distant education from a reptued university for B.Sc and try for M.Sc in IIA and remaining astrophysics universities in india
Mahesh
Do I require GATE score for that?
Lakshmi
Yeah I meañ get a better score in gate.I think the university considers it and would help you to direct enroll into a masters progarmme
Mahesh
But the score should be good.
Mahesh
okay. I will do my best.
Lakshmi
Thank you
Lakshmi
hi guys I am from Ethiopia.I don't know detailedly about astronomy and I want to be astronaut .can you help me guys?
Dawit
I know that astronauts have to be physically fit
Tanmay
I don't know anything else though
Tanmay
If you want to be a astronaut you should complete medicine.
Mahesh
I had done by my b.tech ece long back and then I started doing job...but I still want to switch my field to astronomy, I m confused how shall I pursue it..
Shina
what should we pursue after 12th std B.Sc program and further or through engineering?
Ganesh
Maybe you should ask this question in quora.Professors will tell you answer to the question.
Mahesh
ok
Ganesh

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