Glass is a denser medium(medium in which particles are close to each other) whereas air is rarer medium(medium in which particles are far away from each other). Whenever light rays travel from rarer to denser medium, speed of light decreases. This is because, in denser medium, light rays interact with more particles and due to this there is a time delay. Geometrically,When light rays enter glass or any other denser medium, it bends towards the normal of denser medium because refractive index of glass is more than that of air. Finally, when light ray travels from glass to air, the emergent ray will be parallel(it is not exactly parallel and this is due to curvature of space-time. But the deviation is negligible in all the geometrical optiks experiment) to the incident ray because light ray travels in the same medium i.e air.
