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Comment by mmcconnell1618 | Hacker News Reader

NOVA had a show on medieval stained glass that showed the size of gold particles trapped in glass created different colors. If the mass and electron distribution of gold is solely responsible for its color then shouldn't the size of gold particles (above 1 atom in size) have no effect?

http://www.pbs.org/wgbh/nova/ancient/science-stained-glass.h...

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You're right, the colour of materials is much more complex than just the available atomic transitions. The electronic states are changed and spread out by being bonded to other atoms and you get bulk collective properties like polarizability. The relativistic contraction mentioned in the article is still relevant for the electrons' energy levels in bulk gold (a big lump much larger than the wavelength of visible light) and so neglecting it would give the wrong colour for bulk gold.

Structures up to a few hundred nanometers in size could also conceivably affect the colour. This is kind of like how the atomic-level description of a radio antenna doesn't really matter, more its bulk properties like conductivity. The gold nanoparticle effect is due to Mie scattering (http://en.wikipedia.org/wiki/Mie_Scattering), which is the scattering of light off dielectric spheres approximately the size of the wavelength of light. This effect isn't relevant if you just have a brick of gold.

Indeed. For another example, photos like this are a classic example of how particle size can affect colour:

http://education.mrsec.wisc.edu/background/quantum_dots/imag...

(solutions of CdSe nanoparticles in order of increasing particle size)

Broadly speaking, since "colour" isn't a well defined for things like atoms, but it is for macroscopic objects, it makes sense that there's some weirdness that goes on for particles in between those two extremes.

The color change of gold and CdSe particles with size are due to two different effects.

CdSe is a semiconductor, and reducing the CdSe nanoparticle size increases its band gap through a process called quantum confinement. For smaller particles, it requires a photon to have higher energy (i.e. smaller wavelength) to be absorbed.

Gold particles derive their color from the scattering mechanism mentioned in the parent comment.

Completely agreed; I was just providing another example for how the colour of large objects does not necessarily have anything to do with the colour of small objects. Thanks for the added context :)

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