The normal, angles of incidence, reflection & refraction

- The angles of incidence, reflection and refraction are measured between a ray of light and an imaginary line called the normal.
- In, general terms, if one line is normal to another, then it is at right angles to it.

In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.

- How a normal appears in a geometric drawing depends on the circumstances:
  - - When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
    - When light hits a curved surface, the normal line is drawn straight up from the point where the light hits the surface.
    - If light travels directly through the centre of a sphere, the normal line also passes through the centre of the sphere.
    - When a normal is drawn on a ray-tracing diagram, it provides a reference perpendicular to the surface against which changes in the direction of light can be measured.