At least when I was studying optical engineering, we had to learn (and memorize, which annoyed me greatly) materials properties of different glasses, plastics, some gems like sapphire, etc. Obviously optical properties, but also thermal properties. Maybe some kind of stress thing; I don't remember. But also, we had to have some knowledge of which things cost lots of money. Which things are easier to manufacture with and what sorts of manufacturing techniques work with different materials. etc. I presume a working engineer just knows all of this stuff then.