Refraction of light in a glass block, illustration. A light red (red) from a raybox (grey) passes through a glass block (green) placed on a sheet of paper. Light travels at different velocities in different media, and as it passes from air into the glass at an angle, its speed is decreased, and the direction of travel is bent towards the normal line (a line perpendicular to the air/glass boundary). As the ray emerges its speed increases and it is bent away from the normal. When light passes from air through a glass block that has parallel sides, it emerges parallel to the path of the light ray that entered it.