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?

SInce light slows down in mediums, would it ever be possible to slow light down to a speed where a human could observe its movement?

During a recent lecture on the use of confocal microscopes, we were told that red light penetrates further into tissue than blue and green light.   This was demonstrated by the fact that a red laser is still visible when shone through a finger, but a green one isn’t.   This contradicts the loss of red light that is observed as you descend underwater; everything appears blue at depth when scuba diving.   Why is this?

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.

According to Einstein's famous equation E = MC2, or Energy = Mass times the speed of light times the speed of light. This tells us that anything that has mass, such as matter, is comprised entirely of energy. Any physical object with mass is therefore simply a lump of energy existing in some stable form.

As I understand it, all forms of energy, including chemical energy, kinetic energy and potential energy are covered by Einstein's equation, meaning that fundamentally all forms of energy are the same basic 'stuff' (for want of a better word). My question is, therefore, what exactly is energy? I want to get to a fundamental understanding of energy, because it seems that such an understanding would help to explain much of reality.

I'm not asking for explanations about work done etc, I'm asking at the fundamental level what is energy?

If a space craft uses a laser to push it along in space could it at one point hit the speed of light?

Following the recent spell of cold weather, I noticed that there are still piles of dirty snow with sand or grit (but not salt) in them, but all the clean snow has melted.

I know that light can travel in waves, but what makes up those waves? Is it molecules or what?

I'm short-sighted, and during a recent haircut realised that objects far behind me still looked blurry in the mirror. If the light is reflecting from the mirror (which wasn't far away), why is it blurry?