Similar to the refraction of all waves, light waves change direction slightly as they pass through a different medium. As a ray enters a glass block, it is bent towards the normal. As the ray keaves the block, it speeds up and is refracted away from the normal (at the same angle as when it entered the block). The change in speed causes the ray to change direction.
If the block id perpendicular to the ray of light, the ray is not refracted it just goes straight through on a straight line.
Here is a diagram of refraction:
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