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

Sorry.

Light is energy, which causes excitation of material it hits. Those materials which are "transparent" transmit that excitation more or less "faithfully" from molecule to molecule and then out into the surrounding medium. But that medium (air, in many cases, water...?) is also a material, so it transmits light in exactly the same way.

Now, what about materials which are not "transparent"? What happens when the light energy hits them? A matt black object might warm up a bit. A shiny mirror will reflect - its "surface" molecules transmit the excitation more or less "faithfully" back into the surrounding medium at an angle calculable from the angle at which the light first approached.

And what is the difference between the transmission of light through space (effectively a vacuum?) and through air?

Be careful with assuming everything is made from molecules. A lot of materials - metals for example are not - they are an (effectively) infinite array of atoms. Salts are an array of an ions - glass is an alumino-slicate array - these things are *not* molecular.
Also, most of the space in a substance is just that, space - if it were not it would probably be a neutron star or black hole.

You guys may be interested to know that surface scientists sometimes study surfaces using a technique called RAIRS - Reflectance-Absorbance Infra-red Spectroscopy. Its based on the fact that when light reflects some wavelengths are abosorbed more than others and from this you can compare intensity of wavelengths before and after a reflection - this gives much information on the substances that make up the reflecting surface in question.

Also if we're going to start to talk about excitation or physical interaction between light and other sub-atomic particles we're going to have to discuss the ray/particle duality of light.

Photons!! (as Zaphod would say)

Rama

Sorry I should have said something else there. So what if some things are not molecular I hear you cry. Well the absorbance/remittance of photons is a property of the outer electrons of the substance in question and whether something is molecular with elctrons in discrete molecular orbitals - ionic with elctrons in discrete atomic orbitals (sodium chloride for example) or has an elactron band structure spread over the whole substance like metals and semiconductors is vey important in what happens when a photon is absorbed and reemitted.

Photons are quantised according to the equation E=hf (E = energy, h = Planck's constant f = frequency).
So for something to absorb that photon it must have two electron orbitals of an equivalent enrgy difference (hopefully the lower one being occupied). If there is not an equivalent energy gap anywhere then the light cannot interact with it and moves through it.
Salt (sodium chloride) for example has massive energy gaps between elcetronic orbitals that do not correspond to visible wavelengths of light hence it is transparent to visible light (Yes I know it looks white but a single crystal of it is like glass or quartz - crystal clear). This does not mean it does no absorb any wavelengths of light - it will absorb strongly short wavelenght ultraviolet light as this correspods in energy to the large energy gaps in the electronic orbitals.

A metal has a more or less continuous electron band - this means the elctrons are kind of in a sea - this sea having a continuum of energy in the visible range - hence virtually all wavelengths will be absorbed and because there are no longer discrete energy levels in the elctron orbitals like in a molecule the elctrons decay back to a continous range of energy levels causing a reemittance of light in a more or less continous spectrum again - hence metal is opaque to visible light and highly reflective. Note that the emitting light may not be exactly the same as the absorbed light - hence some metals are coloured such as gold or copper.

Sorry, I've rambled on a bit there

I think I just did Rama

But it explains very nicely why non-metals aren't reflective

The geometry of the light inside the raindrops means that if the light enters in a particular direction, it is reflected back at a particular angle. This occurs 47 degrees (I think, I can't remember the exact number) from the point in the sky directly opposite the sun. This means that the sun is behind you. The lower the sun in the sky, the higher the rainbow. It is possible to have a rainbow just after the sun has set, which will be the highest possible bow, although the bottoms of the two ends will be in shadow just before they touch the ground. No crocks of gold after sunset.

The angle is slightly different for each of the colours, so instead of seeing a narrow band of white, we see a wide band with the colours smeared out.

This describes the primary bow. The secondary bow is caused by the light rays bouncing around the water drop an extra time before coming out. They come out at a different angle, so this bow is in a different place in the sky, further from the point directly opposite the sun. The colours are reversed as well. How many people have ever noticed that?

It is theoretically possible to have a tertiary bow, caused by even more bouncing around inside the raindrop. This would appear in a completely different position in the sky, near the sun, so it is unlikely that it would be visible. I've never heard of any reports of one.

Moon rainbows are caused by the light of the moon on water drops. They are rare but are sometimes seen in the spray around a waterfall. Because it is dark, the eye's colour sensors have shut down, so Moon rainbows appear to be colourless.

Well, I actually thought it went through. How does the molecule know which direction to sedn the light in then? How does it know what is the opposite direction. And as for the reflection, a window also reflects when it is dark on the other side. How?

It doesn't know, it just does.

The window reflects because some of the light hitting it hits certain parts of the structure which turn out to be reflective, while lots of light is still going straight through it (in both directions). You notice it in the dark more because there's no light coming from the other side to overwhelm the reflection.

Absolutely right, Orcus - not everything is molecular or molecular in the same way. It was early in the morning, so maybe I over-simplified.

The point remains, however, that - whatever the atomic, ionic, etc structure - the tiny objects or energy states of which everything is made are not sitting still. Whether you go for a planetary electron model or one of tensions and surges and leaps between energy levels, whether you're talking about crystalline structures, seas of particles in a solid or seas of ions in a fluctuating sea of potential relationships, things are moving, changing, dynamic.

Surfaces are difficult enough. I've never knowingly studied any of this science, but it seems to me that the behaviour of an energy source at the "surface" of a chunk of material is the result of the average of interactions between photons and the particles which make up that surface (and even a surface has a depth).

If a particle/bundle of energy hits a point on a "reflective" surface at an angle of 30 degrees (can't remember the ALTnnn sequence), something says that the average interaction will result in the reflection/re-emission of much or most of the particles/energy at 30 degrees the other side of the perpendicular.

But that effect has to be made up of many, many interactions between one set of "things" which have a direction (the light) and another which are just wriggling round at a low energy level, having settled down to be a solid. How do angles of incidence and reflection result from these individual interactions?

As I said, surfaces are difficult enough. What about materials which happen to be "transparent"? The discussion so far suggests that photons are absorbed by one "level" of atom/molecule/ion/particle and then in some sense re-emitted to the next. How is the "direction" of the light preserved (transparency) or preserved but diverted (refraction like sticking a pencil into water)? Why does everything not just scatter at random?

My brain is beginning to warm up slightly!

Has to do with the frequency of the electron or something. Light is not technically "transmitted" through empty space, it just keeps on going. Think of it sort of like the Pony Express: The rider stops at each station and switches horses. (Consider the horses to be photons.) However, it keeps going in generally the same direction, and it carries the same letter (or wavelength, polarization, etc). Now, in a vaccum, there are simply no stations to stop at. Which explains the slower speed of light in a medium, because it stops at each atom/molecule that it hits before being retrasmitted along its way. Not quite sure how that reflection thing works, though...

I've not studied reflection/refraction in great detail either, but it would seem to make sense that the wavelength of the light has an impact on things, making the calculation of angles a much larger scale affair. An optical microscope has a maximum resolution of around 240 nanometres because the resolution's a function of the wavelength of light. If you assume that an average atom's around 0.1 nanometres in size (which isn't too far wrong), then each photon is interacting with several thousand atoms simultaneously and thus doesn't have to worry about the details, in the same way that a tennis ball doesn't worry about individual specks of dirt on a clay court.

I like the Pony Express analogy, BUT...

light horses travel faster, the further they have to go to the next station (the limitations of an analogy)

the photon is the horse; the direction, colour/wavelength etc of the light is the instruction sitting in the mind of the rider ("get this to Backwoods Gulch"). Where in physics is that described?

Calculation of angles is easy because we can observe what happens. What I'm (suddenly) interested in is the low-level mechanism which conjures this effect out of many, many interactions.

A photon interacts with thousands of atoms simultaneously? How "big" is a photon? Or is it the energy level that matters?

The size of a photon is a question of it´s wave length. But I don´t actually kow if there is an upper limit to that.

So a piece of glass is actually like a one way mirror of sorts?

I´d like to again ask how does the light know how to be absorbed on one side, and go out again on the opposite. And I´m not satisfied with a "It just does". That sounds like religion to me.

The light doesn't "know" anything. It must have properties which, when they interact with the properties of the material the light hits, determine the results - reflection, refraction, heating etc.

"It just does" is an over-simplification, but you can certainly talk in terms of the behaviour of beams of light. As ever, the interactions of single photons and single atoms/molecules/ions of a material seems like a very different thing.

Hell, give me an explanation that requires three doctorates to understand. I´m just not satisfied with a "it just does answer".

I'm afraid "it just does" is the best answer you're going to get at the moment - Heisenberg says you can't find out.

In that case you could have said "I don´t know", that is always a better answer.

I´ve never liked Quantum Mechanics.

I've never liked small people who fix cars.