I saw a question where it supposes a particle of quartz sand with 1 mm in diameter is opaque white. I am then asked on whether this particle is single-crystal, or polycrystal. Admittedly, I am quite stumped here, as the things I've read up on these and heard in the lectures in the class does not go over anything regarding its visible characteristics, including color, size, and the like. I am aware that polycrystals consist of grains due to not having an ordered orientation like that of single crystals, and I am also aware that single crystals have flat surfaces in their geometry. However, I am not sure how these concepts would guide me to the correct answer, or if I am missing essential knowledge regarding how to differentiate them.
Quartz has a band gap of around 9 eV, which corresponds to light being in the UV. Therefore, any light with energies smaller than 9 eV cannot be absorbed by quartz. In other words, quartz should be transparent to any visible light.
Why would quartz ever be opaque in the visible spectrum then? Well, assume the sample is polycrystalline. Then when visible light enters the (poly)crystal (say, front the front), it refracts at various angles as it enters and exits the different grain boundaries. This means some of the light is bent away, and exits through different angles in the crystal. This makes the sample appear opaque.