1.54 Law of Reflection

Transcription

1 1.54 Law of Reflection Define reflection and image. Compare and contrast regular and diffuse reflection. State the law of reflection. 193

2 1.54. Law of Reflection This dancer is practicing in front of a mirror so she can see how she looks as she performs. She s watching her image in the mirror as she dances. What is an image, and how does it get inside a mirror? In this article, you ll find out. Reflected Light and Images Reflection is one of several ways that light can interact with matter. Light reflects off surfaces such as mirrors that do not transmit or absorb light. When light is reflected from a smooth surface, it may form an image. An image is a copy of an object that is formed by reflected (or refracted) light. Q: Is an image an actual object? If not, what is it? A: No, an image isn t an actual object. It is focused rays of light that make a copy of an object, like a picture projected on a screen. Regular and Diffuse Reflection If a surface is extremely smooth, as it is in a mirror, then the image formed by reflection is sharp and clear. This is called regular reflection (also called specular reflection). However, if the surface is even slightly rough or bumpy, an image may not form, or if there is an image, it is blurry or fuzzy. This is called diffuse reflection. Q: Look at the boats and their images in the Figure Which one represents regular reflection, and which one represents diffuse reflection? A: Reflection of the green boat is regular reflection. The water is smooth and the image is sharp and clear. Reflection of the blue boat is diffuse reflection. The water has ripples and the image is blurry and wavy. FIGURE In the Figure 1.124, you can see how both types of reflection occur. Waves of light are represented by arrows called rays. Rays that strike the surface are referred to as incident rays, and rays that reflect off the surface are known as reflected rays. In regular reflection, all the rays are reflected in the same direction. This explains why regular reflection forms a clear image. In diffuse reflection, the rays are reflected in many different directions. This is why diffuse reflection forms, at best, a blurry image. You can see animations of both types of reflection at this URL: http ://toolboxes.flexiblelearning.net.au/demosites/series5/508/laboratory/studynotes/snreflectionmirrors.htm Law of Reflection One thing is true of both regular and diffuse reflection. The angle at which the reflected rays leave the surface is equal to the angle at which the incident rays strike the surface. This is known as the law of reflection. The law is illustrated in the Figure and also in this animation: 194

3 FIGURE FIGURE Summary Reflection is one of several ways that light can interact with matter. When light is reflected from a smooth surface, it may form an image. An image is a copy of an object that is formed by reflected (or refracted) light. Regular reflection occurs when light reflects off a very smooth surface and forms a clear image. Diffuse reflection occurs when light reflects off a rough surface and forms a blurry image or no image at all. According to the law of reflection, the angle at which light rays reflect off a surface is equal to the angle at which the incident rays strike the surface. Vocabulary image: Copy of an object that is formed by reflected or refracted light. law of reflection: Law stating that the angle at which reflected rays of light bounce off a surface is equal to the angle at which the incident rays strike the surface. reflection: Bouncing back of waves from a barrier they cannot pass through. Practice At the following URL, review the law of reflection and watch the animation. Then fill in the blanks in the sentence below When a ray of light strikes a plane mirror, the light ray off the mirror. 2. Reflection involves a change in of a light ray. 195

4 1.54. Law of Reflection 3. The angle of incidence equals the angle between the incident ray and. 4. The angle of equals the angle of incidence. 5. The normal line is to the mirror. Review 1. What is an image? 2. Identify the object and the image in the Figure Which type of reflection formed the image: regular reflection or diffuse reflection? How do you know? FIGURE What is the law of reflection? 5. Label the angle of incidence and the angle of reflection in the Figure FIGURE

5 1.55 Mirrors Explain how mirrors form images. Distinguish between real and virtual images. Describe the images formed by plane, concave, and convex mirrors. What does this picture show? Is it a photo of identical twin sisters, or is it just one girl looking in a mirror? The picture shows a single girl and her mirror image. How Mirrors Form Images A mirror is typically made of glass with a shiny metal backing that reflects all the light that strikes it. When a mirror reflects light, it forms an image. An image is a copy of an object that is formed by reflection or refraction. Mirrors may have flat or curved surfaces. The shape of a mirror s surface determines the type of image it forms. For example, some mirrors form real images, and other mirrors form virtual images. What s the difference between real and virtual images? A real image forms in front of a mirror where reflected light rays actually meet. It is a true image that could be projected on a screen. A virtual image appears to be on the other side of the mirror. Of course, reflected rays don t actually go through the mirror to the other side, so a virtual image doesn t really exist. It just appears to exist to the human brain. Q: Look back at the image of the girl pointing at her image in the mirror. Which type of image is it, real or virtual? A: The image of the girl is a virtual image. It appears to be on the other side of the mirror from the girl. 197

6 1.55. Mirrors Plane Mirror The mirror in the opening photo is a plane mirror. This is the most common type of mirror. It has a flat reflective surface and forms only virtual images. The image formed by a plane mirror is also right-side up and life sized. But something is different about the image compared with the real object in front of the mirror. Left and right are reversed. Look at the girl brushing her teeth in the Figure She is using her left hand to hold the toothbrush, but her image appears to be holding the toothbrush in the right hand. All plane mirrors reverse left and right in this way. The term mirror image refers to how left and right are reversed in an image compared with the object. FIGURE Concave Mirror Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays meet. You can see how concave mirrors form images in the Figure and at the following URL. Concave mirrors are used behind car headlights. They focus the light and make it brighter. Concave mirrors are also used in some telescopes. FIGURE

7 Convex Mirror The other type of curved mirror, a convex mirror, is shaped like the outside of a bowl. Because of its shape, it can gather and reflect light from a wide area. As you can see in the Figure 1.130, a convex mirror forms only virtual images that are right-side up and smaller than the actual object. You can see how a convex mirror forms an image in the animation at this URL: FIGURE Q: Convex mirrors are used as side mirrors on cars. You can see one in the Figure Why is a convex mirror good for this purpose? A: Because it gathers light over a wide area, a convex mirror gives the driver a wider view of the area around the vehicle than a plane mirror would. FIGURE

8 1.55. Mirrors Summary When a mirror reflects light, it forms an image. An image is a copy of an object formed by reflection (or refraction). A real image is a true image that forms in front of a mirror where reflected light rays actually meet. A virtual image appears to be on the other side of the mirror and doesn t really exist. Most mirrors are plane mirrors that have a flat reflective surface. A plane mirror forms only virtual, right-side up, and life-sized images. A concave mirror is shaped like the inside of a bowl. The type of image it forms depends on where the object is relative to the focal point. The image may be real, upside down, and reduced in size; or it may be virtual, right-side up, and enlarged. A convex mirror is shaped like the outside of a bowl. It forms only virtual images that are right-side up and reduced in size relative to the object. Vocabulary concave: Curving inward like the inside of a bowl. convex: Curving outward like the outside of a bowl. image: Copy of an object that is formed by reflected or refracted light. reflection: Bouncing back of waves from a barrier they cannot pass through. Practice At the following URL, review how a plane mirror forms an image, and watch the animation. Then answer the multiple choice questions below A plane mirror forms an image where the reflected rays a. divide. b. separate. c. begin. d. intersect. 2. The image formed by a plane mirror appears to be a. on the same side of the mirror as the object. b. on the opposite side of the mirror from the object. c. either on the same side or the opposite side, depending on the distance of the object from the mirror. d. on both sides of the mirror, regardless of the distance of the object from the mirror. 3. The distance from the object to a plane mirror equals the distance from the mirror to the a. image. b. incident ray. c. reflected ray. d. normal. 4. Anyone who sees the image formed by a plane mirror is sighting at the same image a. size. b. line. c. distance. d. location.

9 Review 1. What is an image? How do real and virtual images differ? 2. Define the focal point of a mirror. 3. Describe the image formed by a plane mirror. 4. What type of image is formed by a concave mirror if the object is between the mirror and the focal point? 5. Mirrors like the one in the Figure are sometimes placed at street intersections so drivers can see around blind corners. What type of mirror is used for this purpose? What type of image does it form? FIGURE