Hacker News Reader

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.

View on HN

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 :)

Reply on news.ycombinator.com