is: a directed line (i.e. a line with an arrow on it) drawn to represent the passage (or potential passage) of light. [P6.1, P6.2]
usually is drawn: at right angles to the wavefront. [P6.1, P6.2]
has direction: which indicates the direction of energy flow. [P6.1, P6.2]
normally is restricted: to situations in which diffraction effects are negligible. [P6.1, P6.2]
Copyright 1997, The Open University