h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I've never liked any small people who fix cars.

Heh - I've been gone for a few days on holiday, so I didn't get back to this stuff until now. Glad to be vindicated (sort of), I hate having to defend myself - particularly when it was all just a misunderstanding. You had me wondering there for a moment, though I was confident since I'd looked it up myself (in several sources) before posting my original message about refraction versus diffraction. I can only stress that a lot of confusion could be avoided if everyone did this.

"It just does!" is like "because I say so." It's not an argument or an explanation: it's a slap in the face, an attempted restoration of order, a punctuation mark.

Most of us, most of the time, would probably like to inhabit a nice Newtonian world where light can be described in terms of straight lines and we don't care how fast it goes and whether it's energy or particles.

If you're looking at a mirror, it's not unreasonable to draw straight lines for the mirror, the light reflected from the extremities of the object you're looking at onto the reflective surface and back to eye. Similarly for a standard, static lens - ignore reflection and consider refraction.

You can think of a rainbow in these simplified terms, but the idea of a giant, annular, reflective/refractive lens which is not the same from one moment to the next or one person to the next is not an easy one to grasp.

When you try to consider how the energy interacts with a "surface" or fights its way through the particles of a material, things get much, much harder.

Consider refraction. Stick a pencil into a glass of water. Observe that it appears to bend at the surface. But what are we actually seeing? We see an object which appears to be "where it is not." What combination of reflection and refraction (Newtonian), what interactions of energies and particles (quantum?) can expain what we see?

Does it matter how much of the material is between the object and the eye, or is the angle of the bend identical for all pencils which extend from a well defined gaseous medium (air) into a well defined liquid medium (water)? If the latter, how do we explain the low-level interactions which produce that result?

Hmmmm. Link between quantum events and macroscopic events - a branch of physics called statistical mechanics - it can be used to derive bulk properties of substances by using statistical dsitribtutions of billions of particular energies etc. Its really rather heavy and mathematical but it does rather neatly show how particles quantised into discrete vibrational, rotational and translational energy levels can collect together in their zillions (a technical term ) to give the newtonian physics we see in the normal macroscopic world.

That is generally how it is done. For example a hydrogen molecule will at a give temperature be in a certain energy level but another at the same temperature may be in a totally differet vibrational and rational state - the probability of a given molecule being in a certain state is dependent on the temperature (a bulk property) as well as the fundamental inherent properties of the molecules itself.
Overall, in 1 mole of hydrogen (the amount that weighs 2 grammes - assuming H2 gas - roughly 6 * 10^23 molecules) the distribution of the molecules in their energy levels is given by the Boltzmann distribution e^-kt e is the base of the natural logarithm (2.18281828...), k is the Boltzman constant (forget its value I'm afraid) and t is temperature.

So the temperature defines the overall distribution of the molcules into the energy levels fundamental to that molecular state. From the Boltzman distribution of the hydrogen molecules one can see why hydrogen has certain spectral properties and can use that information in many different ways.

Hope that wasn't too complicated. This is just an example of how fundamental quantum properties are linked up to bulk properties of substances.

Afraid I don't really know why things are reflected at particular angles. Is it possible that photons do not get absorbed by some surface but simply bounce? I personally doubt it. May be its a statistical ensemble like the Boltzman distribution as mentioned by someone above.