Why is bulk zinc oxide white and the nano form transparent?

[#neutrino]

umm well that's about it

 

[Astronuc]

What does one mean by nano-form? How many atomic layers in the nano-form?

Think about ZrO₂ or TiO₂ and under what conditions they are white or transparent, or even Al₂O₃. Think about crystal vs amorphous vs granular/suspension.

 

[RobS232]

These materials are both transparent in the visible regime of light. However, because they have a higher index of refraction, the light passing through small particles is refracted and reflected off of many surfaces. The resultant scattered light makes them appear white. If the particles are smaller than the wavelength of light, they no longer interfere with its transmission and they are transparent again. They do have absorption bands in the UV regime, and still act as effective sun blocks.