Each of the lines in which light or heat may feel like coming out of the sun or any other luminous body, or even passing through a small opening is a ray. A ray in optics, is defined as idealized model of (https://en.m.wikipedia.org/wiki/Light) light, obtained by choosing a line that is perpendicular to the (https://en.m.wikipedia.org/wiki/Wavefront) wavefront of the actual light, and that points in the direction of energy flow.

IRA is used in X Ray and Optical Spectroscopy.

Thank you.

(https://en.m.wikipedia.org/wiki/Energy_transfer) ^{(https://en.m.wikipedia.org/wiki/Ray_(optics)#cite_note-1)}