>> Really? I thought *all* transparent materials have an IOR different than
>> air. As in, it's impossible to *avoid* this (e.g., if you wanted to make
>> a kind of "invisible glass", you can't do it.)
>
> You can create a cone structure on the surface of the material (with the
> points upwards so it's pointy to touch). Now so long as the size of the
> cones is smaller than the wavelength of light then you get a continuous
> smooth variation of IOR from the air to the solid. This prevents any
> reflections, so the glass looks invisible.

That sounds absolutely trippy - and really expensive to manufacture. ;-)

> There are other anti-reflection coatings already in use, with varying
> degrees of performance. One of them works by destructive interference
> and is used on glasses, outdoor displays etc, you can tell by the
> characteristic purple tint of the remaining reflection (it blocks green
> reflection mainly).

Yeah, interference is wavelength-specific, so you can only make stuff 
non-reflective at one wavelength. I've seen Falstad. ;-)

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