Normal line - Flat surfaces
We have later a great way to remember the normal line. But for now we will just explain what the normal line is in relation to rays of light entering a flat surface.
The normal line is an imaginary line perpendicular (at 90°) to where the ray of light touches the boundary.
For a ray of light travelling from air to water the refraction would be:
and for a ray of light travelling from water to air the refraction would be: