Basics of optics

In short materials and environments (air, vacuum, water, glass, diamonds) that don't conduct electricity are transparent if they don't have cracks or scrapes. Materials that conduct electricity (metals, graphite and pure silicon) are nontransparent and they can reflect light. For example thin metal foil can reflect light even if that traveled several light-years through space and atmosphere. In addition light reflects partially between areas that have different electric conductivity like between air and water or air and random nonconducting solid.

Cracks can make otherwise transparent materials like glass nontransparent by making it move semi-randomly in many directions. If light enters material with different refraction index and different local speed of light then it bends somewhat and cracks
For transparency material shouldn't have cracks and scrapes larger than the wavelength of visible light (around 0,5 micrometers).


Light sound waves visible light waves also bend around things that are shorter than wavelength so within couple of wavelength waves straighten out and continue behind small objects. That's also the reason why radio waves used in radars have to be smaller than object to detect it and x-rays with wavelength similar to molecules can make molecular structure visible.