Colour and Interstellar Clouds
The earliest evidence for the existence of interstellar grains can be seen in irregular dark regions where dust-bearing clouds obscure the light from the stars behind them. Perhaps the most famous example of this phenomenon, known as interstellar extinction, is the "Horse Head" nebula. This particular nebula is an example of a dark region superimposed upon a bright nebula.
The extinction of starlight by interstellar dust is caused by a combination of absorption and scattering, where light photons effectively bounce of dust particles. When this occurs the wavelength of the light remains constant but the direction is altered. This process works more efficiently at shorter wavelengths which means that blue light is scattered more easily than red light, therefore the stars appear to be reddened. Because the extinction of short wavelength starlight is so severe ultraviolet telescopes are of little use for observing these clouds.
The light emitted from stars is unpolarized, however the light that we observe can show a polarization of up to 10%. The amount of polarization observed is directly proportional to the amount of interstellar reddening. This shows us that the interstellar dust clouds must be at least partly responsible for the polarization of starlight. >From this we can suggest that these interstellar dust grains must be non-spherical and in some way aligned, hence not randomly arranged.
A reflection nebula occurs when an interstellar dust cloud lies to one side of a star rather than in-between the star and the observer. By the same phenomenon that brings about interstellar reddening, light is scattered towards the observer. As above, blue light is scattered much more readily than red light so these clouds appear bluer to the observer. The light observed from these Reflection Nebulae is polarized but this is due to reflection polarization rather than transmission polarization, which again infers properties of interstellar dust clouds. Below is an example of a reflection nebula, called IC353