Question

A ______ is one narrow beam of light traveling in a straight line.

Answer 1

In optics a ray is an idealized model of light, obtained by choosing a line that is perpendicular to the wavefronts of the actual light, and that points in the direction of energy flow. Wikipedia

Answer 2

A light ray is one narrow beam of light traveling in a straight line. It is the most basic element in geometrical optics. A light ray is an idealized and hypothetical model of a light that is used to better explain the propagation of light. In reality, light travels as a wave and can therefore be bent.