I know when to use the words glass and cup in reference to a drinking container but had never realized that part of the definition concerned whether it was opaque or translucent. You learn something new all the time.

a cup is a concave vessel for drinking from, usually made of opaque material
She poured the coffee into the cup.
The lemonade filled the paper cup to the brim.
Styrofoam cups insulate you hand from the hot chocolate.

a glass is a vessel from which one drinks, especially one made of glass, plastic, or similar translucent or semi-translucent material.
You are usually served a glass of ice water in a restaurant.
He watched the chocolate syrup swirl as it was stirred into the milk in the glass.