Normal

If one line is normal to another, then it is at right angles. In geometry, the normal (or a normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.

Light and geometry
  • Light travels in a straight line through a vacuum or a transparent medium such as air, glass, or still water.
  • When light encounters an obstacle or passes from one transparent medium to another, it can result in a variety of optical phenomena including absorption, dispersion, diffraction, polarization, reflection, refraction, scattering or transmission.
  • Geometry can be used to calculate the outcome of light encountering different optical phenomena.
  • When a normal is drawn on a ray-tracing diagram, it provides a reference against which changes in direction of light can be measured.
Drawing a line normal to another
  • When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to, so at a right angle (900) to the surface.
  • When light strikes a curved surface or plane, the normal is drawn at a tangent to the surface.
  • When light strikes a sphere, the normal is drawn from the centre of the sphere and through the strike-point on the circumference.
References
  • https://en.wikipedia.org/wiki/Geometrical_optics