The word “photonics” is derived from the Greek word “photos” meaning light. It covers all technical applications of light over the whole spectrum. Most applications, however, are in the range of the visible and near infrared light. With a brief history of classical work and tenets of optics, we will present electrical circuit of a thin-film device used in a fuel cell, called Alkali Metal Thermo Electric Convertor (AMTEC).The device uses infrared radiation to knock out electrons from some alkali metal, which after going through a circuit and having done the prescribed work, meet the ions again. The system is closed and continues working as long as the radiation source is kept on. The longevity, power and efficiency of the device depend inversely to some extent on the thickness of electrodes used for collecting electrons freed from the alkali metal, as well as of the solid electrolyte. The details of the device’s circuit comprising both electrical and optical functions will be discussed.