Normal

• The way in which 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.

Remember that:

• Light travels in a straight line through a vacuum or a transparent medium such as air, glass, or water that is still or in a constant state of motion.
• 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, such as the angle of refraction or reflection.
• When a normal is drawn on a ray-tracing diagram, it provides a reference perpendicular to the surface against which changes in direction of light can be measured.