Optical and electromagnetic fundamentals are presented in this fourth unit of the Project Physics text for use by senior high students. Development of the wave theory in the first half of the 19th Century is described to deal with optical problems at the early stage. Following explanations of electric charges and forces, field concepts are introduced in connection with electrons, currents, potential differences, Oersted's discovery, Ampere's work, and moving charges in magnetic fields. Faraday's lines of force are used to analyze electromagnetic induction and its applications as well as modern civilization under the influence of scientific discoveries. Further discussions of field theories are made for electromagnetic radiation, taking into account Maxwell's theories, Hertz's experiment, wave propagation, electromagnetic spectra, and ether concept. Historical developments are stressed in the overall explanation. Problems with their answers are provided in two categories: study guide and end of section questions. Also included are related illustrations for explanation purposes and a chart of renowned people's life spans between 1700 and 1900. The work of Harvard Project Physics has been financially supported by: the Carnegie Corporation of New York, the Ford Foundation, the National Science Foundation, the Alfred P. Sloan Foundation, the United States Office of Education, and Harvard University. (CC)