Light entering glass from air bends toward the perpendicular, which shows that glass is optically:

1. Bending direction reveals which medium is slower. 2. At a non-normal incident angle, the path of the light ray bends. 3. It bends closer to the perpendicular in the optically slower substance. 4. Here the ray bends toward the perpendicular on entering glass. 5. So glass is the optically slower substance. _Source: OpenStax College Physics for AP Courses 2e (CC BY 4.0), Ch 25 'Geometric Optics', sections 25.1-25.7_