Double-A wrote:

(G=EMC^2 Glazier) wrote in message ...
Well I'm looking at my red sun glasses,and the reason they are red is
only red light can go through the glass. What if space has this same
feature??? Bert

That's an interesting thought, but I believe they can tell by the
shifting of spectral lines for elements such as hydrogen in the light
that it has been Doppler shifted and not merely filtered through red
space dust.

Certainly, and the Doppler shift of each component could likely be identified.

Suppose we're looking at a star that's receding from us: the bright
emission lines from excited atoms in its atmosphere will appear at
slightly lower frequencies than in a standard spectrum, all of them
by the same amount. Now suppose that the star had gone through an
episode of coughing up red dust -- however unlikely that may be -- in
an expanding cloud whose trailing edge (that we have to look through
to see the star) is actually approaching us. We must be calling the
dust "red" because it absorbs a lot of blue and green light, showing
a spectrum with dark bands in those regions, so we'd only be able to
see the emission lines in the red area, or elsewhere between gaps in
the pattern. But this "reddening" is quite independent of the
red-shift in the bright spectrum. Anyway, assuming that we can
identify the 'signature' of the absorbing material, we'll see the
dark bands at slightly *higher* frequencies than normal. From this we
could deduce that the dust is approaching us while the star is receding.

This may be a rather silly example, but the principle that different
sets of spectral features can show different red- or blue-shifts is
important in the study of close binary systems, stars with turbulent
atmospheres, planetary nebulae (& other kinds), star-systems
containing disks or vortices of gas, and other situations where
various different types of material are in motion.

--
Odysseus