# As the waves travel through a medium, they lose velocity

1. Apr 30, 2007

### Big-J

as the waves travel through a medium, they lose velocity, however the frequency is constant right?

thanks

Last edited: Apr 30, 2007
2. Apr 30, 2007

### Claude Bile

Frequency remains constant - wavelength and velocity change when moving from one medium to another.

(Also, I think you mean medium, not median.)

Claude.

3. Apr 30, 2007

### Staff: Mentor

Waves travel at a constant velocity as determined by the properties of the medium.

4. Apr 30, 2007

### Big-J

Thanks for the help guys, but I don't get how a wave can have a constant velocity...if it eventually stops moving.

5. Apr 30, 2007

### Staff: Mentor

As long as the medium doesn't change, the speed of the wave doesn't change. The amplitude does decrease, either because the energy in the wave is spreading out over a larger region (for example, a light wave spreading out in all directions from a light bulb), or because of frictional losses in the medium.

6. May 1, 2007

### rcgldr

I thought there was a theory that there was some red shift (lowering of frequency) when light traveled very long distances ...

7. May 1, 2007

### Repetit

I believe you are talking about the Doppler effect? This is not due to light traveling long distances, but due to the object emitting light moving away from you.

8. May 1, 2007

### rcgldr

No, something about the light from far away galaxies exhibiting a red shift not accounted for by speed alone.

9. May 1, 2007

### christianjb

I'm surprised no-one has mentioned dispersion.

http://en.wikipedia.org/wiki/Dispersion_(optics [Broken])

Last edited by a moderator: May 2, 2017
10. May 1, 2007

### Staff: Mentor

That sounds like "tired light", which I'm not sure I'd call a theory...

11. May 1, 2007

### Reshma

Gravitational red shift?

12. May 1, 2007

### Sojourner01

Consider a wave in the sea. Which direction is the water moving? As a wave loses energy, the water 'wiggles' less and less, but the speed at which it transfers this 'wiggle' to the 'chunk' of water next to it doesn't change. The velocity of a wave is not the same as the speed of the stuff that's 'waving'.

13. May 1, 2007

### rcgldr

Could be, it's been a long time since I read the article.

14. May 1, 2007

### jambaugh

You can build a perfectly good theory in which "tired light" is present.
Take a Special Relativity along with a slight change in the geometry of space-time, e.g. use a fixed deSitter space-time and you get that the Poincare group is now SO(4,1). Note that this is *not* an expanding universe cosmology.
But all translations are pseudo-rotations when seen by observers far from the points orthogonal to the axis of pseudo-rotation. (Just as from the poles someone walking along the equator is really "rotating").

In such a theory light from distant sources necessarily redshifts and so distant objects appear to be accelerating away, indeed are accelerating away. Nonetheless the spatial universe doesn't grow in size over time.
I know it sounds paradoxical but it does work and is locally consistant with SR.

Note also this is distinct from the actual deSitter cosmology which is of an expanding universe and wherein the SO(4,1) symmetry is not asserted to correspond everywhere with the local Poincare group.

Regards,
James Baugh