Normal

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 geometric optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), 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.
  • Geometric optics treats light as a collection of rays that travel in straight lines and calculates the effect of different optical phenomena.
  • The normal is a line drawn on a ray diagram which provides a standard reference against which the deviation of a ray of light can be calculated when optical phenomena such as reflection or refraction take place.
  • The normal can be established for any transparent material by laying a sample onto a sheet of paper which has a straight line drawn across it. A laser torch can then be shone along the line and the position of the material adjusted until laser and line match.

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