Working through this chapter of the study guide will enable you to:

1. Distinguish between diffuse and regular reflection and tell when each will occur.
2. Explain how light is refracted when it penetrates a new medium and how this can lead to the formation of a spectrum of colors.
3. See how light can be polarized and what useful things can be done with polarized light.
4. Understand the interaction of two or more light waves as they are added together and how this can cause both constructive and destructive interference.
5. Learn under what conditions light can be defracted around sharp edges or through narrow slits.
6. Show how the reflection of light from the surfaces of plane and spherical mirrors can produce real and virtual images.
7. Determine how lenses can focus light rays and how they can be used to correct vision defects.

Discussion

Now that we know what waves are and how they behave, we can turn our attention to some of the effects they produce in our everyday lives. We use light as the primary source of input to our senses. The interaction of light waves with our surroundings is, therefore, something that is crucial to our understanding of the world around us. The law of reflection not only explains how we can see images in the smooth surfaces of plane mirrors but also the way light bounces off rough surfaces in an irregular way, which allows us to see nearly all of the objects around us.
Because light is a wave, it follows the rules of wave addition generally referred to as superposition. This means two light waves that are in phase can produce a brighter overall intensity when they strike the same area of a screen; or if they are out of phase, they can cancel each other, which results in reduced illumination. These constructive and destructive effects explain patterns produced by the bending of light around sharp edges and the colors seen in soap bubbles. The transverse wave nature of light also means that it can be polarized by reflection and by transmission through suitable filter materials. Polarization can be used to reduce glare from reflected sunlight and thus make it easier to see and safer to drive on bright, sunshiny days.

Section  7.1 Reflection


When light strikes most surfaces, it reflects; that is, at least part of the incoming electromagnetic wave energy bounces back in the general direction from which it originally came. If the surface is very smooth, the reflected waves produce a clear image of the object from which the light originated. This is called regular reflection. On the other hand, if the surface is rough, the pattern of reflected light is irregular and no image can be seen. This is called diffuse, or irregular reflection. In this latter case, however, the diffusely reflected light allows us to visualize the surface itself and thus be aware of its structure and location. If it were not for this effect, most of us would walk around bumping into things much of the time.
Reflection occurs when light interacts at a media interface and rebounds from that surface. The law of reflection states that no matter at what incident angle a ray of light strikes such a surface, the light ray will always reflect from that surface at the same angle. The angle at which the original light ray strikes the surface is called the incident angle, and the angle at which it rebounds is known as the reflected angle. Both of these angles are measured with respect to a perpendicular line drawn to the surface, which is called the normal line. This law is valid for all light rays reflected from both smooth and rough surfaces.
The paths followed by rays of light can be very useful in predicting and explaining the effects produced by various optical processes. Ray diagrams are often drawn to help us visualize the paths followed by light during reflection and also during refraction. We will study refraction in some detail in the next Section . The simple but exact rules that must be followed to produce accurate ray diagrams for various optical devices are also covered in later Section s of this chapter. Development of scaled ray diagrams can be a great help in visualizing the way that light rays behave when both lenses and mirrors are used to achieve desired optical effects.

Section  7.2 Refraction and Dispersion


The path followed by a light ray can be changed as the light ray enters a new medium. This occurs because the speed of light is generally not the same in different transparent materials. The bending of light, called refraction, is responsible for the distortion of objects seen below the surface of a lake or swimming pool, the focusing of light by glass or plastic lenses, and also for the splitting of white light into splendid rainbows of color. Refraction of a light ray traveling from air into an optically denser medium causes the ray to be bent toward the normal line. This means that the angle of refraction is smaller than the angle of incidence in such cases.
When a light ray passes from a dense optical medium into a less dense optical medium, it is bent away from the normal line. This can occur only up to a certain incident angle called the critical angle. If the angle of incidence becomes greater than the critical angle, the ray cannot leave the material at all, and the ray is totally reflected back into the original substance through which it was traveling. (See Figs. 7.10, 7.11, and 7.12 in the textbook.) This phenomenon is called total internal reflection. Total internal reflection explains many interesting and useful processes, such as why some cut gemstones are more brilliant than others and how light can be transported through transparent fiber-optic elements over long distances without substantial losses in light intensity.
The index of refraction (n) is defined as a ratio of two speeds - the speed of light through a vacuum divided by the speed at which light travels through any other transparent material under study. Table 7.1 in the textbook lists the indexes of refraction for several common substances. Notice that since light travels fastest through a vacuum, the index of refraction is always greater than 1.00. But it is never greater than 2.42, the value for the index of refraction of diamond, which has the highest n of any known transparent substance. A material with a large index of refraction is said to have a high optical density; a substance with an index of refraction near 1.00 has a low optical density.
Dispersion of white light occurs because the various colors of light have slightly different indexes of refraction and so are bent at somewhat different angles when white light, which is composed of all colors, passes through a prism or diffraction grating. This results in the formation of a continuous color spectrum producing all of the visible colors: red, orange, yellow, green, blue, and violet, as shown in Fig. 7.13 and 7.14 in the textbook. Dispersion is also responsible, together with total internal reflection, for the formation of several spectacular atmospheric phenomena involving color, the most common of which is the rainbow.

Section  7.3 Polarization


Normal light is usually unpolarized. Light can be either partially polarized or linearly (totally) polarized depending on whether its electric field vectors are ordered preferentially in a partial or complete way. Linearly polarized light has all of its electric field vectors aligned in a single plane. Even though the human eye cannot detect polarization, this condition has several useful characteristics that can be applied to everyday optical processes.
The wave nature of light explains how light can be polarized by reflection, by scattering, and by transmission through a suitable material such as a Polaroid H filter or a natural tourmaline crystal. Polarization can be used to reduce the direct glare produced by the reflection of sunlight from non-metallic shiny surfaces. Glasses with polarized lenses make it easier to see brightly lighted objects on sunny days when such direct reflection is likely to produce unwanted glare.

Section  7.4 Diffraction and Interference


Light waves can be deviated from their initial direction of travel when they pass near sharp edges or through narrow slits. This "deflection" of light is called diffraction. Diffraction is a fairly subtle effect and is not easily noticeable, but close observation of the light passing sharp edges shows a distinct pattern of bright and dark regions that are formed by this optical process. These bands of light and dark are called a diffraction pattern. Diffraction was one of the first conclusive experimental observations showing that light has wavelike properties.
Because light is a wave, it follows the rules of superposition just like the mechanical waves that we studied in Chapter 6. This means that two light waves that are in phase produce a region of brighter overall intensity when they strike a suitable screen. Alternatively, if the two light waves are out of phase, they cancel each other. This cancellation results in dark spots where the two waves meet. Constructive and destructive effects also explain the bright spectral colors seen in soap bubbles, mother-of-pearl, and peacock feathers, and other iridescent substances.

Section s 7.5 Spherical Mirrors


Spherical mirrors use the principle of reflection to focus light rays. Mirrors can be studied by using ray drawings and by applying the spherical mirror equation to the optical processes under study. Each method has some advantages as a learning tool, but each will give the same description of the orientation and properties of the object and image formed by a mirror. Use the textbook to study these important techniques for understanding the optical properties of a curved mirror.
The reflection of light from curved mirrors allows us to manipulate light and thus control the formation of useful optical images. Understanding how light interacts with curved mirror surfaces enables us to build projectors so that images can be shown to many people at once, design powerful telescopes that allow us to see distant or extremely faint objects more clearly, and focus light into parallel beams like those produced by the headlights of a car.

Section s 7.6 Lenses


The direction in which a light ray is traveling can sometimes be changed as it enters a new medium because the speed at which light travels does not remain the same in all materials. This leads to the bending of light rays as they pass into the new material in a process known as refraction. Lenses rely on the refractive properties of materials having different indexes of refraction and varied shapes to control the path that light rays follow. One of the most important uses of lenses is the design and construction of eyeglasses and contact lenses to correct distorted vision. The refraction process also explains the operation of light pipes or optic fibers when light is traveling through a transparent material with a higher index of refraction than the surrounding medium and strikes the inside of the light pipe at an angle greater than the critical angle.
The effect that a lens has on light passing through it can be studied by using the thin-lens equation and by doing ray tracing on a scale model of the lens system under study. Each ray of light from an illuminated object can be precisely traced as it is deflected by a lens, and these paths can be used to predict the location and properties of the image formed in this refraction process. Carefully study the details of this ray tracing process in the textbook. The insight that it gives you can be quite helpful in understanding the way that lenses aid us in correcting defective vision and focusing light where we need it to be concentrated.

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