什么情况下glass用单数

glass 表示玻璃，玻璃杯的时候是单数，但当glasses 意思眼睛的时候，当作复数。

例句

A glass of water is on the desk.

There is a glass of water on the desk.

The window glass is shining under the sun.

He always wears glasses.

When the sun shines brightly， people usually wear sun glasses.