IPA: /oʊˈpeɪkli/
KK: /oʊˈpeɪkli/
In a way that is not transparent or clear; not allowing light to pass through.
The glass was treated so that it could be seen only opaquely.
Opaquely → It is formed from "opaque" (from Latin "opacus", meaning shaded or dark) and the suffix "-ly" (meaning in a manner of). The word "opaquely" means in a manner that is not able to be seen through or is dark.
Think of something that is 'dark' or 'shaded' ('opacus') and how it can be described as being done 'in a manner' ('-ly'). This helps you remember that 'opaquely' refers to something that cannot be seen through.
No commonly confused words.