In Chapter 2 we were acquainted with the behaviour of light, mainly in terms of its wave and particle nature, which are the subjects of physical and quantum optics respectively. This chapter is largely concerned with geometrical optics, the part of optics dealing with light's propagation in terms of rays, i.e. how it travels in straight lines, also known as rectilinear propagation. The concept of light rays is helpful in studying the formation of images by lenses. Further, in many light-related phenomena, such as reflection, refraction and transmission, the properties of light are altered according to different physical properties of the optical media with which they interact. This is the subject of photographic optics in this chapter. Finally, the photometry of image formation will be discussed at the end of the chapter.