Re: [Phys-l] Nano-Particle: Color

[Brian Whatcott <betwys1@sbcglobal.net>]

Tony wrote:
> I was just reading that when the particles of gold  are 100nm it appears
> yellow. But when the particles are 25nm, the color changes to red. This is
> the way medieval artisans made red stained glass. My question is by what
> mechanism do these smaller particles produce this color?
> -Tony
>   
I sprang to the thought that for a light spectrum in the range 400-700nm,
particles smaller than a wavelength would have diminishing power to affect
as a proportion of their size compared with the wavelength of the 
incident light.
So I initially expect  25 nm particles to look "biggest" to the  
shortest wavelengths - i.e blue light - and preferentially scatter this 
color.  
If this sort of explanation were TRUE, it would be confirmed by a side 
view of a flask of colloidal gold appearing blue, while the direct view 
would be red.
 You recognize that this is a version of the "Why is the sky blue" story.

I fear that this story is inappropriate however : I seem to recall that 
early glass can be viewed  "color-fast" at a range of angles, and I 
await a better response....

Brian W