Refraction – water or glass to air
When light travels at an angle from water or glass to air, the light speeds up and changes direction. The light turns away from the normal line.
Think of it as a tractor moving from a slow muddy, grassy field onto a fast-moving road.
The first wheel hits the fast road and starts to speed up. The other front wheel is still slowed by the sticky mud. This makes the tractor change direction.
Light rays travel at 226,000km/s through water but when the light reaches the air its speed will return to 300,000km/s.
The same effect occurs from glass to air.
When light enters a less dense substance (with a lower refractive index) it bends away from the normal line.