Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Blue, Red shift

  1. May 7, 2003 #1
    Can anyone explain what is blue shift and red shift?? And what effects they have?
  2. jcsd
  3. May 7, 2003 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    blue shift and red shift are the astronomers' words for
    frequency shifts up or down
    blue shifted light is light with the frequency shifted up
    (by motion towards, doppler-fashion)

    redshifted light is light with the frequency shifted down
    (either by motion away, doppler, or by the expansion of
    the space in between, or by a gravity effect)

    the light from each kind of atom has a distinctive recognizable pattern of spectral lines, so if you see some hot hydrogen gas you can recognize that it is hydrogen and you can tell (from measurements in the laboratory with the same gas) what
    frequencies it OUGHT to be emitting.

    So if you measure the light that is actually getting to you and find that it is redshifted by ten percent (frequency ten percent lower, or wavelength ten percent longer) then you say "z = 0.1"

    z is the conventional symbol for the redshift, the fractional (increase in wavelength, or) decrease in frequency. So a ten percent decrease in frequency means "z = 0.1".

    And that corresponds to the doppler effect of motion away by approximately one tenth of the speed of light.

    Or a ten percent stretchout of space during the time the light was on its way to you.

    There are some fine points and an improved doppler formula and some ambiguities about what is relative motion away and what is the expansion effect, but basically the redshift z = 0.1 would be interpreted as nearly all due to a tenpercent expansion of space during the light's passage and it would be informally talked about as if it were due to the emitter moving away at one tenth the speed of light.
    Last edited: May 7, 2003
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook