Chapter 13

1. Chapter 13 Section 2 Flat Mirrors

2. Distinguish between specular and diffuse reflection of light. • Apply the law of reflection for flat mirrors. • Describe the nature of images formed by flat mirrors. Objectives

3. Let’s suppose you have just had your hair cut. What do you want do ? • Answer: see the back of your head • You can do this task by using two mirrors to direct light from behind your head to your eyes. • Redirecting light with mirrors reveals a basic property of light’s interaction with matter. Reflection of Light

4. Light traveling through a uniform substance, whether it is air, water or a vacuum, always travels in a straight line. • However, when light encounters a different substance, its path will change. • Reflection is the change in direction of an electromagnetic wave at a surface that causes it tomove away from the surface. Reflection of light

5. Reflection of light (and other forms of electromagnetic radiation) occurs when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface. The simplest example of visible light reflection is the surface of a smooth pool of water, where incident light is reflected in an orderly manner to produce a clear image of the scenery surrounding the pool. Throw a rock into the pool (see Figure 1), and the water is perturbed to form waves, which disrupt the reflection by scattering the reflected light rays in all directions. Reflection

6. Reflection

7. Some of the earliest accounts of light reflection originate from the ancient Greek mathematician Euclid, who conducted a series of experiments around 300 BC, and appears to have had a good understanding of how light is reflected. However, it wasn't until a millennium and a half later that the Arab scientist Alhazen proposed a law describing exactly what happens to a light ray when it strikes a smooth surface and then bounces off into space. Law of reflection

8. The incoming light wave is referred to as an incident wave, and the wave that is bounced away from the surface is termed the reflected wave. Visible white light that is directed onto the surface of a mirror at an angle (incident) is reflected back into space by the mirror surface at another angle (reflected) that is equal to the incident angle, as presented for the action of a beam of light from a flashlight on a smooth, flat mirror in Figure 2. Thus, the angle of incidence is equal to the angle of reflection for visible light as well as for all other wavelengths of the electromagnetic radiation spectrum. This concept is often termed the Law of Reflection. It is important to note that the light is not separated into its component colors because it is not being "bent" or refracted, and all wavelengths are being reflected at equal angles. The best surfaces for reflecting light are very smooth, such as a glass mirror or polished metal, although almost all surfaces will reflect light to some degree. Law of reflection

9. The amount of light reflected by an object, and how it is reflected, is highly dependent upon the degree of smoothness or texture of the surface. When surface imperfections are smaller than the wavelength of the incident light (as in the case of a mirror), virtually all of the light is reflected equally. However, in the real world most objects have convoluted surfaces that exhibit a diffuse reflection, with the incident light being reflected in all directions. Many of the objects that we casually view every day (people, cars, houses, animals, trees, etc.) do not themselves emit visible light but reflect incident natural sunlight and artificial light. For instance, an apple appears a shiny red color because it has a relatively smooth surface that reflects red light and absorbs other non-red (such as green, blue, and yellow) wavelengths of light. The reflection of light can be roughly categorized into two types of reflection. Specular reflection is defined as light reflected from a smooth surface at a definite angle, whereas diffuse reflection is produced by rough surfaces that tend to reflect light in all directions (as illustrated in Figure 3). There are far more occurrences of diffuse reflection than specular reflection in our everyday environment.

10. The texture of a surface affects how it reflects light. • Diffuse reflectionis reflection from a rough, texture surface such as paper or unpolished wood. • Specular reflectionis reflection from a smooth, shiny surface such as a mirror or a water surface. Types of reflection

11. The angle of incidence is the the angle between a ray that strikes a surface and the line perpendicular to that surface at the point of contact. • The angle of reflection is the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves. • The angle of incidence and the angle of reflection are always equal. Law of reflection

12. Angles of reflection

13. To visualize the differences between specular and diffuse reflection, consider two very different surfaces: a smooth mirror and a rough reddish surface. The mirror reflects all of the components of white light (such as red, green, and blue wavelengths) almost equally and the reflected specular light follows a trajectory having the same angle from the normal as the incident light. The rough reddish surface, however, does not reflect all wavelengths because it absorbs most of the blue and green components, and reflects the red light. Also, the diffuse light that is reflected from the rough surface is scattered in all directions. Types of reflection

14. Perhaps the best example of specular reflection, which we encounter on a daily basis, is the mirror image produced by a household mirror that people might use many times a day to view their appearance. The mirror's smooth reflective glass surface renders a virtual image of the observer from the light that is reflected directly back into the eyes. This image is referred to as "virtual" because it does not actually exist (no light is produced) and appears to be behind the plane of the mirror due to an assumption that the brain naturally makes.

15. Flat mirrorsform virtual images that are the same distance from the mirror’s surface as the object is. • The image formed by rays that appear to come from the image point behind the mirror—but never really do—is called a virtual image. • A virtual image can never be displayed on a physical surface.

16. Light and reflection video Video

17. Today we learned about flat mirrors • Next class we are going to learn about curved mirrors Closure