| Topic: |
Science > Physics |
| User: |
"Danny" |
| Date: |
03 May 2006 08:37:02 AM |
| Object: |
Q Re light |
This is a multi-part message in MIME format.
------=_NextPart_000_0008_01C66E8C.B4595CF0
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
My background is not physics, I'm just trying to understand something...
On the one hand, I understand that the "Doppler" effect with respect to =
light waves (I don't know the correct term) causes a shift in the light =
spectrum (for example galaxies may appear "red shifted" or "blue =
shifted" depending on whether they are going away from or coming toward =
earth). But on the other hand, Einstein's Special Theory of Relativity =
says (?) that the speed of light is the same for all observers =
regardless of their motion relative to the source of light. =20
These two concepts sound contradictory - if the speed of light is the =
same for all observers then why the shift in spectrum depending on =
whether the object is approaching or receding? Thanks for your help.
Danny
------=_NextPart_000_0008_01C66E8C.B4595CF0
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=3DContent-Type content=3D"text/html; =
charset=3Diso-8859-1">
<META content=3D"MSHTML 6.00.2900.2873" name=3DGENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>My background is not physics, I'm just =
trying to=20
understand something...</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>On the one hand, I understand=20
that the "Doppler" effect with respect to light waves (I don't =
know=20
the correct term) causes a shift in the light spectrum (for example =
galaxies may appear "red shifted" or "blue shifted" depending on whether =
they=20
are going away from or coming toward earth). But on the other =
hand,=20
Einstein's Special Theory of Relativity says (?) that the =
speed=20
of light is the same for all observers regardless of their motion =
relative to=20
the source of light. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>These two concepts sound contradictory =
- if=20
the speed of light is the same for all observers then why the shift =
in=20
spectrum depending on whether the object is approaching or=20
receding? Thanks for your help.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Danny</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV></BODY></HTML>
------=_NextPart_000_0008_01C66E8C.B4595CF0--
.
|
|
| User: "Sam Wormley" |
|
| Title: Re: Q Re light |
03 May 2006 09:10:39 AM |
|
|
Danny wrote:
My background is not physics, I'm just trying to understand something...
On the one hand, I understand that the "Doppler" effect with respect to
light waves (I don't know the correct term) causes a shift in the
light spectrum (for example galaxies may appear "red shifted" or "blue
shifted" depending on whether they are going away from or coming toward
earth). But on the other hand, Einstein's Special Theory of Relativity
says (?) that the speed of light is the same for all observers
regardless of their motion relative to the source of light.
*Relative motion* between light source and observer that causes the
Doppler Effect
http://scienceworld.wolfram.com/physics/DopplerEffect.html
http://hyperphysics.phy-astr.gsu.edu/Hbase/relativ/reldop.html
The speed of light remains constant. Think of "the wavelength stretching
or compressing" while the wave speed remains constant.
These two concepts sound contradictory - if the speed of light is the
same for all observers then why the shift in spectrum depending on
whether the object is approaching or receding? Thanks for your help.
Danny
.
|
|
|
|
| User: "PD" |
|
| Title: Re: Q Re light |
03 May 2006 08:44:37 AM |
|
|
Danny wrote:
My background is not physics, I'm just trying to understand something...
On the one hand, I understand that the "Doppler" effect with respect to light waves (I don't know the correct term) causes a shift in the light spectrum (for example galaxies may appear "red shifted" or "blue shifted" depending on whether they are going away from or coming toward earth). But on the other hand, Einstein's Special Theory of Relativity says (?) that the speed of light is the same for all observers regardless of their motion relative to the source of light.
These two concepts sound contradictory - if the speed of light is the same for all observers then why the shift in spectrum depending on whether the object is approaching or receding? Thanks for your help.
Danny
Two comments:
1. Even in sound, the Doppler shift due to a moving source has nothing
to do with a change of speed of sound in the medium for the observer.
The speed of the wavefronts as they pass the observer is the same as it
always was, but the space between the wavefronts changes due to the
motion of the source.
2. The Doppler shift for light is not quite the same as the Doppler
shift for sound, at least as normally taught in textbooks. In
particular, the case of an observer moving through the sound medium has
a particular physical explanation that does not pertain to light
(because there is no medium).
PD
.
|
|
|
|
|
Related Articles |
|
|
