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Thanks Patrick for your explanations and patience, I have been reading some old American Scientist issues, before discarding the less interesting, and I gather that greater extinction of blue light or uv by interstellar dust is due to both absorption and scattering (1977; p.448) and the author deduced typical particle size on this basis. Your explanation helps to make sense of this. But I wonder how expansion of space time can be reconciled with Planck's constant and c. If wavelength is stretched by an expansion of space time then frequency must decrease as you say. Otherwise the product of frequency and wavelength would exceed c. And if this expansion decreases frequency is there not some mysterious loss of energy ? Or is the expansion of space time enabled by that lost energy ? When light of some wavelength is absorbed, by a particle or substance, and is subsequently emitted will this usually imply an increase in wavelength ? Are spectral lines also shifted by absorption/emission ? Dave On 2019-04-07 2:56 p.m., Patrick Kelly wrote: > Rayleigh scattering affects blue light more than red light because tiny dust particles and many molecules are closer in size to the wavelength of blue light than that of red light (the wavelength of red light is about twice as long as that of blue light) so they interact predominantly with blue light. (That is why radio waves travel through smoke and fog....they have REALLY long wavelengths compared to particle size). Thus light from the setting sun allows red light through while scattering blue light in random directions making the sun or moon appear reddish when near the horizon as the light has to travel through a lot of atmosphere. Similarly, the white sunlight passing overhead has the blue light scattered in random directions, including downward, resulting in a blue sky. > > In the case of the cosmological redshift, caused by the expansion of the universe, the wavelength of the light is stretched as new space-time is created. Shifting to a longer wavelength means that blue light is shifted from the blue to the red part of the visible spectrum. Note that the term “redshift” means shifted to longer wavelength which does not always mean the light becomes red. There are quasars whose light is redshifted so much by the cosmological red shift that some spectral lines that are normally found in the ultraviolet are shifted into the blue part of the spectrum, and some spectral lines are shifted from the red into the infrared! > > Pat > > Sent from my iPad > >> On Apr 7, 2019, at 12:17 PM, Burkhard Plache <burkhardplache@gmail.com> wrote: >> >> Hi Dave, >> your last question makes the correct observation that of all the light >> emitted by a distant object, the shorter wavelength light will be >> preferentially scattered away, leaving more of the original red light >> than blue light arriving at our doorsteps. Your implied, though not >> stated, assumption seems to be that redshift is measured by 'relative >> amounts of light' or 'the light looking more red'. - However, redshift >> is measured by looking at spectral lines, which are not modified by >> Rayleigh scattering. - Could you clearly state what your argument is? >> Thanks, >> Burkhard >> >>> On Sun, Apr 7, 2019 at 10:53 AM David Webster <dwebster@glinx.com> wrote: >>> >>> >>>> On 2019-04-07 8:45 a.m., Burkhard Plache wrote: >>>> Hi David, >>>> to correct a common misrepresentation: The cosmological red shift of >>>> light is not due to the source moving away but due to the space >>>> expanding. Two very different phenomena. >>>> Also, Rayleigh scattering is not changing the wavelength of the >>>> scattered light, hence is not contributing to redshift. >>>> Burkhard >>>> >>>>> On Sun, Apr 7, 2019 at 8:17 AM David Webster <dwebster@glinx.com> wrote: >>>>> Hi Burkhard, >>> Thanks. It seems to me that we are getting tangled up in semantics. >>> >>> If space expands then it makes objects appear to be moving away. >>> >>> And, indeed, scattering does not destroy shorter wavelengths but it >>> does deflect them so they are partially or entirely culled from those >>> waves which are moving from source to observer. Thus, at the local >>> level; blue skies, white clouds, red sunsets and that green flash which >>> one sometimes sees from the cockpit when landing and facing west near >>> sunset. >>> >>> The above are all effects of our atmosphere. But there is ample >>> evidence of cosmic dust, ranging from particles to atoms, so one would >>> expect scattering of shorter wavelengths throughout space to increase >>> with distance between observer and source; greater opportunity for >>> scattering. >>> >>> So rephrasing my question in current jargon, are red shifts of >>> light due to expansion of space, distinct from red shifts which might be >>> due to Rayleigh scattering whereby shorter wavelengths from a source are >>> less likely to reach an observer ? >>> >>> Or more directly, why is the observed increase in red shift with >>> distance between source and observer attributed to an expansion of >>> space as opposed to greater opportunity for scattering of shorter >>> wavelengths of light as this distance increases ? >>> >>> Dave >>> >>> >>> >>> >>>>> Dear All, but especially astrophysics experts, >>>>> >>>>> Is the red shift of light, which would be due to the source moving >>>>> away at great speed, intrinsically unlike the red shift due to Rayleigh >>>>> scattering (which selectively scatters shorter wavelengths; 1/[length to >>>>> the fourth power]) ? >>>>> >>>>> With ample dust in space, ranging from particles to atoms, one >>>>> would expect the red shift due to scattering to also be a function of >>>>> distance to source. >>>>> >>>>> Dave Webster, Kentville >>>>>