## Lines normal to one another

###### About lines that are normal to one another
• If one line is normal to another, then it is at right angles.
• In geometry, a normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
• In the field of optics, the normal is a line drawn on a ray-tracing diagram perpendicular to (at 900 to), the boundary between two media.
• If the boundary between two media is curved then the normal is drawn at a tangent to the boundary.

## The normal, angles of incidence, reflection & refraction

###### About the normal, angles of incidence, reflection & refraction

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 direction of light can be measured.