ice is a crystal, and as long as the crystal structure in the ice is even, the light passing through it is mostly the same direction and color as when it entered (small amounts are reflected away and filtered). but even with impure ice, you can see the white clouding inside caused by the crystal structure not forming evenly, as the structure likely started crystallizing from each impurity in many different directions.
in the same way, shaved ice and snow are smaller crystal structures that are no longer aligned, so the light passing through them is refracted many times, with each crystal acting like a mini prism. the whiteness is what diffused light looks like (scattered and originating from many original directions) so any view to the other side is obscured.
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