When light that is traveling through air hits a glass surface at an angle, it changes its course. Similarly, when light that is transmitted through glass hits air at an angle, it changes course again. Optometrists use these two facts to bend light so that it is better focused for our eyes. The cornea does something similar for our optic nerves, but sometimes our eyes are imperfectly shaped, become inflexible due to aging, etc. So, we need the assistance of glasses to improve on what our eyes do naturally.