# Do light waves change over distance within a vacuum?

1. Mar 23, 2009

### xcrunner2414

Specifically, does the amplitude of a wave of light change if it does not encounter any matter?

2. Mar 23, 2009

### cragar

we can have a gravitational redshift were light will change wavelength
moving away from a stong gravitational field ,

3. Mar 23, 2009

### xcrunner2414

So, if the universe consisted of a uniform spherical mass and light, when the light waves emanate radially away from the mass would the light waves decrease in amplitude or only wavelength? What exactly happens?

4. Mar 23, 2009

### cragar

k i know that when the light travels outward into a weaker gravitational field
the wave-length becomes longer and the frequency increases but im not sure
about amplitude

5. Mar 23, 2009

### HallsofIvy

As a spherical wave spreads out, without touching anything, the total energy in it does not change. However, it is spread over a greater area (the area of a spherical surface is proportional to the square of the radius) so the intensity at any point on that surface decreases as the inverse square.

6. Mar 24, 2009

### MATLABdude

Over a great enough distance (and we're talking cosmological distances here--millions of light years, i.e. the distance between galaxies), the wavelength gets longer as well. Owing to the expansion of the universe, cosmological redshifting (as opposed to regular redshifting from receding objects) occurs.
http://en.wikipedia.org/wiki/Cosmological_redshift#Redshift_velocity_and_recessional_velocity

The planned James Webb telescope is an IR telescope because the "first light" (emitted when the universe was a wee billion or two years old) has travelled so far that it's been stretched out and is now in the infrared.
http://en.wikipedia.org/wiki/James_Webb_Telescope

...But for the most part (on earth or within the solar system, for instance), the wavelength doesn't change to any measurable extent.

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