Refraction of light through a glass slab occurs when light passes from air into the glass, slowing down and bending toward the normal line at the surface. Inside the glass, the light travels more slowly in a straight line until it reaches the opposite surface. As it exits back into the air, it speeds up again and bends away from the normal. This bending of light causes objects viewed through the glass slab to appear slightly raised or shifted, as the light takes a different path than it would have in air.