SNC2D PHYSICS 4/27/2013. LIGHT & GEOMETRIC OPTICS L Plane Mirrors (P ) Plane Mirrors. The Laws of Reflection. i r

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1 SNC2D PHYSICS LIGHT & GEOMETRIC OPTICS L Plane Mirrors (P ) Plane Mirrors Your dental hygienist needs to see the back of your teeth in order to clean them properly. To do this, they use a small round mirror at the end of a long handle. This mirror actually goes inside your mouth. Without this mirror, it would be very difficult for your hygienist to do their work. How does the mirror allow your hygienist to see the back of your teeth? April 27, DPHYS - Plane Mirrors 1 The Laws of Reflection Mirrors reflect light in predictable ways. As the angle of reflection (r) changes, so too does the angle of incidence (i). In fact, the two are always equal to one another. In addition, the incident ray, reflected ray, and normal are all on the same plane (flat surface). These observations are known as the laws of reflection. i r April 27, DPHYS - Plane Mirrors 2 1

2 The Laws of Reflection LAWS OF REFLECTION Î the angle of reflection equals the angle of incidence ( 2 r = 2 i ) Ï the incident ray, normal, and reflected ray all lie in the same plane i r April 27, DPHYS - Plane Mirrors 3 Reflection You know that light is reflected by smooth, shiny surfaces such as mirrors. Light is also reflected off rough surfaces, such as paper, concrete, and clothing. We know that rough surfaces reflect light because we can see them. So, why can we see a clear reflection of ourselves in a mirror but not in concrete? April 27, DPHYS - Plane Mirrors 4 Regular Reflection When parallel rays of light from an object strike a very smooth surface, such as a mirror, the reflected rays move away from the mirror parallel to each other. This behaviour of light against a smooth surface is called regular reflection. April 27, DPHYS - Plane Mirrors 5 2

3 Regular Reflection REGULAR REFLECTION the reflection of light off a smooth surface reflected rays move away parallel to each other can see a clear image April 27, DPHYS - Plane Mirrors 6 Problems With Regular Reflection Normally, the surface of a road is rough. On a wet road, water can fill in the spaces between the bumps and smooth out the road surface. This is why a wet road sometimes produces regular reflection. Drivers sometimes experience this reflection as a glare, especially at night. April 27, DPHYS - Plane Mirrors 7 Problems With Regular Reflection People with dyslexia have difficulty reading print. Many people who are dyslexic complain about the glare off white paper: there is too much reflected light from the paper. The contrast between the white paper and the black text makes reading difficult. April 27, DPHYS - Plane Mirrors 8 3

4 Diffuse Reflection A mirror has a very smooth surface. In contrast, a dull surface, such as concrete, has many small bumps in it. When parallel rays of light from an object strike the bumpy surface, the rays strike the bumps at different angles of incidence. Therefore, the reflected rays bounce off the bumps at different angles. This is called diffuse reflection. We are not able to see clear reflections in rough surfaces because of diffuse reflection. April 27, DPHYS - Plane Mirrors 9 Diffuse Reflection DIFFUSE REFLECTION the reflection of light off a rough, irregular surface reflected rays bounce off the rough surface at different angles unable to see a clear reflection April 27, DPHYS - Plane Mirrors 10 Applications of Regular & Diffuse Reflection Human-made materials are often designed so that the way they reflect light matches their use. For example, vehicle dashboards are bumpy so that any reflection is diffuse, which is less distracting to the driver. Many cyclists, runners, and walkers wear safety strips that produce regular reflection to make them more visible at night. April 27, DPHYS - Plane Mirrors 11 4

5 When you see an object reflected in a mirror, you do not see the object itself. Instead, you see an image of the object. An image is a picture of the object produced by light. IMAGE a picture of an object formed using light rays mirrors make the light rays meet, or appear to meet April 27, DPHYS - Plane Mirrors 12 Scientists use four characteristics to describe a reflected image: size, attitude, location, and type. April 27, DPHYS - Plane Mirrors 13 SIZE Is the image larger, smaller, or the same size as the object? April 27, DPHYS - Plane Mirrors 14 5

6 ATTITUDE Is the image upright or upside down? April 27, DPHYS - Plane Mirrors 15 LOCATION Does the image appear behind, in front of, or on the surface of the mirror? April 27, DPHYS - Plane Mirrors 16 TYPE Is the image virtual or real? A real image is formed when light rays actually meet at the image. A virtual image is an image formed in a location where light does not actually reach. The light only appears to come from this point. April 27, DPHYS - Plane Mirrors 17 6

7 NOTE! You can remember the four characteristics of an image from the acronym SALT (Size, Attitude, Location, Type). April 27, DPHYS - Plane Mirrors 18 IMAGE CHARACTERISTICS (SALT) size (larger, smaller, same) attitude (upright or upside down) location (position relative to the mirror surface) type (virtual or real) REAL IMAGE formed when light rays actually meet (i.e. real rays) can be seen (captured) on a screen VIRTUAL IMAGE formed where light rays do not actually reach (i.e. virtual rays) cannot be seen on a screen (must look into the device) April 27, DPHYS - Plane Mirrors 19 Activity: Locating Images Using Laws of Reflection INTRODUCTION An image in a plane mirror is always a virtual image. It looks as though it is behind the mirror. The only way to see a virtual image is to look into the device. You can use a ray diagram and the laws of reflection to explain why. April 27, DPHYS - Plane Mirrors 20 7

8 Activity: Locating Images Using Laws of Reflection INSTRUCTIONS A. Incident rays from the object hit the surface of the mirror and are reflected back to the observer (2 r = 2 i ). NOTE! These incident or real rays are shown as solid lines that extend outward from the object. April 27, DPHYS - Plane Mirrors 21 Activity: Locating Images Using Laws of Reflection INSTRUCTIONS B. The reflected rays enter the observer's eyes. An image of the object then forms on the part of the eye called the retina. April 27, DPHYS - Plane Mirrors 22 Activity: Locating Images Using Laws of Reflection INSTRUCTIONS C. The observer s brain then interprets the location of the image as behind the mirror. Because of the travels-in-a-straightline property of light, the brain unconsciously extends the light rays through the plane mirror forming a virtual image of the object behind the mirror. April 27, DPHYS - Plane Mirrors 23 8

9 Activity: Locating Images Using Laws of Reflection NOTE! These extended or virtual rays are shown as dashed lines that extend back from the reflected rays to the virtual image. April 27, DPHYS - Plane Mirrors 24 Activity: Locating Images Using Laws of Reflection QUESTIONS 1. Where will the image of the pen appear in the following? April 27, DPHYS - Plane Mirrors 25 Activity: Locating Images Using Laws of Reflection RECALL! The light rays behind the mirror are drawn as dashed lines. This indicates that these rays do not really exist they are virtual. And since the rays are virtual (dashed) so too is the image. April 27, DPHYS - Plane Mirrors 26 9

10 Activity: Locating Images Using Geometry INTRODUCTION You can also locate an image in a plane mirror by using geometry. April 27, DPHYS - Plane Mirrors 27 Activity: Locating Images Using Geometry INSTRUCTIONS A. A straight line is drawn from the tip of the object, perpendicular to the mirror, and extended behind the mirror. This is called an objectimage line. April 27, DPHYS - Plane Mirrors 28 Activity: Locating Images Using Geometry INSTRUCTIONS B. The distance from the object to the mirror (ie the object line) is measured. April 27, DPHYS - Plane Mirrors 29 10

11 Activity: Locating Images Using Geometry INSTRUCTIONS C. The image line is made the same length as the object line. The end of the image line is the location of the virtual image. April 27, DPHYS - Plane Mirrors 30 Activity: Locating Images Using Geometry NOTE! If you draw object-image lines from enough points on an object, you can locate the object s virtual image. April 27, DPHYS - Plane Mirrors 31 Locating Images IMAGES IN PLANE MIRRORS are always: the same size as the object upright located behind the mirror virtual April 27, DPHYS - Plane Mirrors 32 11

12 U Check Your Learning 1. Why is the word ambulance spelled in reverse on the front of an ambulance vehicle? April 27, DPHYS - Plane Mirrors 33 U Check Your Learning 2. In your own words, state the laws of reflection. refer to your notes April 27, DPHYS - Plane Mirrors 34 U Check Your Learning 3. If you can see someone s face in a flat mirror, they can see your face. Use a diagram and the laws of reflection to explain why. April 27, DPHYS - Plane Mirrors 35 12

13 U Check Your Learning 4. How is an image seen in a plane mirror similar to the actual object? How is it different? similarities same size and orientation differences location (in front/behind) and type (real/virtual) April 27, DPHYS - Plane Mirrors 36 U Check Your Learning 5. You have been asked to design the interior of a new speedboat. Do you think you should choose a smooth or rough surface for the dashboard? Why? rough surface so reflection is diffuse and less distracting to driver April 27, DPHYS - Plane Mirrors 37 U Check Your Learning 6. How would you arrange two mirrors in order to see the back of your head. Make a sketch of the arrangement. Be sure to include the incident and reflected rays. (Note: if mirrors are available, try this activity.) April 27, DPHYS - Plane Mirrors 38 13

14 U Check Your Learning WIKI(PHYSICS) O... 2DPHYS - QUIZ1 (Light & Reflection) April 27, DPHYS - Plane Mirrors 39 14