Obviously we can all see full spectrum light when it's pointed directly at us or when it's reflected/refracted off of an object. However if a light (a torch say) was shined horizontally across our field of vision, from an angle where we could not observe the light directly. Across a vaccum where the beam of light would not be interfered with or interrupted whatsoever. Would we be able to observe the beam of light. If so, how? And if so, wouldn't space be light instead of black?

I am exploring the properties of waves when traveling thorough a transparent medium. I observed that when white light is shone through a prism the blue light refracts more than the red light in the spectrum. This is due to the interaction between the blue light and the molecules within that medium. 

I wish to know if anyone can give me more details the nature of this interaction.

A more through explanation as to why wavelength of the light effects the speed through a medium would be appreciated and any relevant laws or theories as this is a mystery to me.