Normal line – flat surfaces
Later on, we'll show you a great way of remembering the normal line. But for now, we will just explain what the normal line is in relation to rays of light hitting 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 look like this:
And for a ray of light travelling from water to air the refraction would be: