If one line is normal to another, then it is at right angles. So in geometry, the normal is a line drawn perpendicular to and intersecting another line.

In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (90^{0}), to the boundary between two media.

- Light travels in a straight line through a vacuum or a transparent medium such as air, glass, or still water.
- If light encounters a force, an obstacle or interacts with an object, a variety of optical phenomena may take place including absorption, dispersion, diffraction, polarization, reflection, refraction, scattering or transmission.
- Optics treats light as a collection of rays that travel in straight lines and calculates the way in which they change direction (deviate) when encountering different optical phenomena.
- When the normal is drawn on a ray diagram, it provides a reference against which the amount of deviation of the ray can be shown.
- The normal is always drawn at right angles to a ray of incident light at the point where it arrives at the boundary with a transparent medium.

https://en.wikipedia.org/wiki/Geometrical_optics