16.1 Reflection

• In all these cases, light is modeled as traveling in a straight line, called a ray.
• Light may change direction when it encounters the surface of a different material (such as a mirror) or when it passes from one material to another (such as when passing from air into glass). It then continues in a straight line—that is, as a ray. 
• Because light moves in straight lines, that is, as rays, and changes directions when it interacts with matter, it can be described through geometry and trigonometry. This part of optics, described by straight lines and angles, is therefore called geometric optics.
• There are two laws that govern how light changes direction when it interacts with matter: the law of reflection, for situations in which light bounces off matter; and the law of refraction, for situations in which light passes through matter. In this section, we consider the geometric optics of reflection.
• The law of reflection states: The angle of reflection, 𝜃rθr, equals the angle of incidence, 𝜃iθi . This law governs the behavior of all waves when they interact with a smooth surface, and therefore describe the behavior of light waves as well. The reflection of light is simplified when light is treated as a ray.
• Light reflected in this way is referred to as specular (from the Latin word for mirror: speculum).
• Because the light is reflected from different parts of the surface at different angles, the rays go in many different directions, so the reflected light is diffused.
• An image in a mirror is said to be a virtual image, as opposed to a real image. A virtual image is formed when light rays appear to diverge from a point without actually doing so.
• Some mirrors are curved instead of flat. A mirror that curves inward is called a concave mirror, whereas one that curves outward is called a convex mirror.
• Ray diagrams can be used to find the point where reflected rays converge or appear to converge, or the point from which rays appear to diverge. This is called the focal point, F.
• The distance from F to the mirror along the central axis (the line perpendicular to the center of the mirror’s surface) is called the focal length, f.