Presented is book four in a series of six books in the University of Illinois Astronomy Program which introduces astronomy to upper elementary and junior high school students. This document terms the analysis of light as an essential clue to understanding astronomical phenomena. Topics discussed include: thm behavior of light; the wave model and the particle model of light; the electromagnetic spectrum; the role of spectra in determining stellar temperature, size and chemical composition; the origin of light and the Bohr model of the atom; and the Doppler effect as an aid to understanding the motion of stars and galaxies. (Author/DS)