The issue is how fully you want transparency and opaqueness to be manifest. The only truly transparent medium is a vacuum. And an opaque medium is anything that blocks/reflects back the light radiation. This includes most many items, from solids to liquids to even gases. However, there are things that are sufficiently transparent or opaque that they will do for the particular purpose under consideration. So even if those items are in actuality translucent, they may be sufficiently one extreme or the other that they allow the function to continue. Such as a window, camera lens, or your cornea. These are transparent enough. A sleep mask, isolation chamber, or your window blinds may be opaque enough.

As I tell my students when discussing the properties of minerals, transparent and opaque represent a range of possibilities. If something is transparent, then light will pass through such that you can see what is on the other side. For example, if I turn on the light inside of a room, someone looking through a normal pane of glass can see me inside of the room. Then there is translucent. Light is scattered such that you can't actually see through the object but you can see the light coming through it. An example of this would be if I replaced the glass in my window with frosted glass. Someone outside would be able to tell if there was a light on in the room, but would not be able to see me or what I was doing. Then we get to the other extreme, opaque. Light does not pass through something that is opaque. So, if I were to brick up my window, then someone standing outside would not be able to tell if the light was on inside or not. I wrote above that these terms represent a range of possibilities. With many substances, the ability to transmit light will depend on how thick the substance is. If the substance is thin enough, then it will be transparent. However, as the sample gets thicker, transparent will grade into translucent and ultimately into opaque. Some other substances will be opaque no matter how thin you slice them. So, the answer to this question really depends on the substance. Let me give you some examples. The mineral muscovite (http://webmineral.com/data/Muscovite.shtml) will show a range of transparency from transparent to opaque depending on how thin it is. On the other hand magnetite (http://webmineral.com/data/Magnetite.shtml) will be opaque even if the sample is just just 0.03 mm thick. (Hair ranges from about 0.25 to 0.1 mm.)