Absorption and scattering
Interstellar gas and dust are not perfectly transparent. Absorption transfers radiation energy into matter, while scattering redirects it; both can reduce the direct beam along a line of sight.
Scattered light can also be detected elsewhere. X-ray dust halos, described by MPE, provide an example in which the surrounding scattered radiation and spectral absorption carry different information about intervening material.
References: [1]
Extinction, reddening and redshift
Across many optical and near-infrared conditions, dust suppresses shorter wavelengths more strongly, dimming and reddening background light. This changes relative band intensities rather than shifting every spectral feature to a longer wavelength.
Reddening and redshift must therefore be distinguished. One concerns the altered distribution of received light; the other measures a feature’s wavelength displacement. Spectra and environmental information help distinguish them.
Obscuration as evidence
Barnard 68 obscures a background star field in visible light, while near-infrared observations reveal more background sources. Their colour changes allow a map of obscuration and the cloud’s material distribution.
Extinction is consequently both an observational obstacle and evidence. Stellar distance or temperature estimates must account for it, while dust estimates from background stars need reference colours and propagation assumptions.