What happens to emission spectra when photons pass through a medium?

Endervhar

I understand that light travels at “c” through any medium, and that the apparent slowing results from the absorption and re-emission of the photons by atoms in the medium.

If this is the case, why, for example, are astronomers able to see the emission/absorption spectra of distant stars? Why are these spectra not converted into the emission spectra of the atoms in the lenses of their telescopes?

NB. the use of stellar spectra is by way of an example. This is not a question about astronomy. :)

mfb

In lenses, the wavelength is different from the wavelength in a vacuum. That does not matter. The lenses (or mirrors) do not have spectral lines in the observed wavelength range. For observations in the infrared spectrum, they might need cooling to avoid radiation from the instrument itself.

Cthugha

Unfortunately, this is pretty much what is not happening. However, this misconception is so common that we already have a FAQ entry on that here:http://www.physicsforums.com/showthread.php?t=511177.

Maybe that helps a bit. Feel free to ask if some additional questions arise from reading the FAQ.