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

Energy, including light, is transmitted in discrete 'packets' or 'quanta' - hence quantum physics (I think this was the inspiration of Neils Bohr) In terms of light the quata are photons, neither particle or wave but both, and in the case of white light composed of all frequencies in the visible spectrum, mixed.

When white light falls upon a coloured surface, the white light is absorbed BUT the specific wavelength of the coloured surface, excites the atoms (of which the coloured surface is made up - and from which the colouration is derived on an atomic level. These atoms 'emit' a photon in response, equal to the wavelength of the absorbed light that originally excited it.

It is this 'new' emitted photon which is ultimately intercepted by the retina of an eye and triggers the psychological perception of colour.

In short, although we talk normally of light reflecting off surfaces, and in the case of colour of specific wavelengths of lights reflecting off surfaces am I right to think that the actually quanta of energy packets behave in the way I've described, because ultimately all surfaces are composed of atoms.

Thanks.

This is for the introduction to an entry I am writing where I'm aiming to describe how colour vision works, and how colourblindness is a malfunction of that.

I don't think that that can be an adequate description of reflection, because an emitted photon would go in a random direction, while a reflected light beam obeys strict rules as to exactly which direction it is reflected in.

Really? Hmm (why would a photon go in a random direction?)


Okay I'll accept that I was trying to remember what I could of what Bohr said about photons and atoms.

So where is my error?

I think when white light strikes a polished coloured surface, some light is reflected back by the surface, and some is absorbed by the atoms in the surface and re-emitted at the same wavelength, and some is absorbed and not re-emitted, causing the surface to heat up slightly. The colour of the surface is determined by the wavelengths that are re-emitted, but the reflected light is a different thing again. This is why we can have shiny black cars.

Does that make sense?

Ye-e-e-e-s. (pause while I think about it )

So since I'm writing about colour - and accepting what you say about reflecting and scattering is correct - when you said >>some is absorbed by the atoms in the surface **and re-emitted at the same wavelength**, and some is absorbed and not re-emitted, causing the surface to heat up slightly.<<

MY description fits that part of the process but just not the reflecting of the light.

So the photons that are re-emitted - what are they?

BTW Gnomon - just found your article on colour, I shall have to link to it.

I haven't finished that article yet.

But I don't fully understand this. Get a blue biro and fill in a 1cm square with it. What colour is the square? Blue. But tilt the page until the light from the light above you is reflected off the square and what colour is the light? Not white but red.

I tried it out. Couldn't get it to work*. (Where do you hold the paper?)

Best guess, I'm reading up seperately on consciousness (Dan Dennet) and he uses several examples of false-colour reports where the brain processes something into vision (and hence consciousness) that isn't there.

This may be another example of that phenomenon and not anything to do with the physics of the light but rather the psychology of the viewer**


*and I'm not sure I'd 'see' red

**but then again if it's a psychological effect - who the heck knows?

No, this isn't a psychological effect, or a brain processing effect. I understand about those.

I reckon you could measure the red using a spectrometer. It works better with an ink pen. YOu need to hold the paper up near the light and tilt it so that the "shine" of the light on the paper is on the inky bit. You'll see the red colour, although it is not amazingly noticeable.

I've been thinking more about this. The colour of something is the light that is absorbed and re-emitted. Because particular atoms or molecules can only emit certain frequencies, you only get some colours of light off the object, and those combine to give you the colour of the object. Something which emits both red and blue light, but not green light, for example, has the colour of "magenta".

I found some text books online which are proving useful in sorting out the differences between colour as a property of objects, and colour as perceived, and the neural triggers that underlie the experience of colour by the triggering, in different combinations, the relative sensitivities of the photoreceptive cones.

I've decided to ditch my current draft and do it over from scratch.

1st drafts are there to be amended. I've realised I need to work up a better grasp of colour theory before I start taking it apart explain how colourblindness is a malfunction of it.

>>The colour of something is the light that is absorbed and re-emitted. Because particular atoms or molecules can only emit certain frequencies, you only get some colours of light off the object, and those combine to give you the colour of the object. Something which emits both red and blue light, but not green light, for example, has the colour of "magenta".<<


Yes and in the case of, say, a purple flower, the emmitted frequencies would be whatever it takes make purple (probably not as simple as blue and red but for arguments sake) the blue and red light is reflected, but because I am a protanope and do not detect the red light I just see the purple flower as blue.

I believe I understand most of the aspects of colour, and always intended to write a series of articles about it, but never got around to it.

Some things worth knowing:

1.

Red light and Green light mixed together look like yellow light to a person with normal colour vision, but they are not. Shining the mixture into a prism, it will produce a red component and a green component. Shining the mixture into a prism will produce just yellow light.

That's all withing the Newtonian theory of colour.

2.

Illuminate a small object with red light from one side and green light from 90 degrees around from that. You'll get two shadows, one red, one green. Straightforward and explainable by the Newtonian theory.

Illuminate a small object with red light from one side and white light from 90 degrees around from that. You'll get two shadows, one red, one green. Not straightforward and and not explainable by the Newtonian theory.

Lookup Edwin Land and the Retinex theory of colour vision for more details.

Hi Clive,

There is something that I've never been clear on. An atom, say on the surface of something, absorbs a quanta of energy and then re-emits it. I can see how it would send the same packet as came in but I can't see how it could send some other colour as happens with fluorescence. Ultraviolet is absorbed but visible light is reflected.

Fluorescence is much more complex.

Atoms and molecules abosorbing light in the UV/visible region of the eletromagnetic spectrum will have their electrons excited into a higher energy level.
The fate of this extra energy underlies all these things.

The extra energy can split chemical bonds, it can be dissipated as vibrations, rotations and translational energy (heat) and it can be released straight back again as the electrons return to their original configration and pop a new photon out of the same frequency.

Fluorsescence occurs when a process called intersystem crossing happens.

The excited state, instead of doing all of the above somehow changes into another excited electronic state but with different configuration of electrons. This relaxes down to it's lowest vibrational energy state - hence losing energy through vibrational, rotational and translational transfer to other molecules.
The new excited state - of lower energy than the original - then relaxes back to the first state of the molecule before light was shone on it (the ground state). The photon emitted though is now of a different (lower energy) to the incident light.

This is normally a 'forbidden' process as it breaks all sorts of rules that must be obey when a molecule is excited by a photon (photons have certain types of momentum that must be conserved in the process). These rules are strict in small, very symmetrical molecules but more relaxed in larger systems where there are many closely overlapping excited stated - when there are a large number of these then their 'density' means that the rate of intersystem crossing can overcome the slow rate of 'forbidden' processes.


When intersystem crossing leads to a requirement to break a really strict 'forbidden' process then we get a really slow leakage of the excited state back to the ground state which can keep happening long after the incident light is switched off. This is the closely related process of phosphorescence.

Sorry I have two process named the same thing. My terminology is a bit askew in the above.

The conversion of an excited electronic state to another of lower energy - leading to fluorescent behaviour is termed 'internal converstion'.

Intersystem crossing - leading to phosphorescence - is a form of internal conversion that leads to a spectroscopically forbidden state.

I'm sure that's as clear as mud now

Thanks Orcus, it is clearer now. I knew that something more that mere reflection was going on, I just didn't know what it was.

I have a thread going on in SEx with a question about SETI. Maybe you could take a crack at that.

Well I could but I can't say I know much about the SETI project. It's not something that excites me and - for me - there are more interesting problems in science that are available to be solved using spare processing power over the internet.

Orcus, the reason that I asked you was that my question over there was more about the physics of radio transmission and that looked like what we were discussing here.

Quick thing about the Blue ink square reflecting red:

Doesn't this illustrate the difference between the colour of an object and what it reflects....

If an object is not "shiny" (i.e. reflective) it simply absorbs the light impinging on it, and emits light in the wavelength of whatever colour it is.

If it is imperfectly reflective it absorbs some of the light and reflects some of the light. So when you look at it from an angle where most of the light is NOT reflected (e.g. straight above?), it will look like it is the "colour" of the ink. If you look at from an angle where most of the light is reflected light, you will see that light that is NOT absorbed by the pigment (if it is blue I guess you will see Red).

Perfectly reflective would, at a guess, be a perfect mirror, and wouldn't have a colour of its own?