Glass may have different compounds added to it to achieve a desired effect. Typcially, soda-lime glass has a refractive index around 1.5, meaning that light travels through it at two-thirds the speed it does through a vacuum.
It depends on the type of glass you have, for example Crown glass has an index of 1.50 unlike the Flint glass which has an index of 1.91. It's all about the glass.