When visible light enters a different medium from one of lower refractive index, it slows down, i.e. when light passes from air into glass. If it enters the medium at an angle to the two media's boundary, the angle between the light path and the boundary will decrease as the light slows, known as refraction. The opposite effect occurs when the light then leaves the higher refractive index medium, producing a higher angle between the boundary and light path.
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