Images in Plane Mirrors Chapter 10, Lesson 3

Learning Goal To learn how to use light rays to locate an image in a PLANE MIRROR Understanding how to communicate effectively the reflection of an image in a plane mirror

Introduction When you are standing in front of a mirror in the bathroom while brushing your teeth, you may notice that the writing on your T-shirt seems backwards Or maybe you have always wondered why the writing on the hood of a police car or ambulance appears backwards when standing in front of it Well, this all has to do with reflections in plane mirrors

Introduction

Using Light Rays to Locate an Image Light rays and the laws of reflection help determine how and where an image is formed in a plane mirror A light source gives off millions of light rays in all directions, but we only care about the rays that actually strike the mirror and are reflected into your eyes

Using Light Rays to Locate an Image These rays are reflected off the mirror, with the angle of incidence being equal to the angle of reflection

Using Light Rays to Locate an Image From everyday experience, we know that light travels in a straight line. When your eyes detect reflected light from a plane mirror, your brain projects these light rays backwards in a straight line This results in your brain thinking that there is a light source behind the mirror and that this is where the light rays originate

Using Light Rays to Locate an Image

Using Light Rays to Locate an Image It is this apparent light source behind the mirror that results in you seeing an image behind the mirror. There isn’t a real light source behind the mirror because the mirror is opaque.

Using Light Rays to Locate an Image This kind of image is called a virtual image A virtual image is an image in which light does not actually arrive at or come from the image location The light only appears to come from the image Your eyes detect the light rays, but your brain determines where the image is located.

Using Light Rays to Locate an Image

Using Equal Perpendicular Lines You can use light rays and the laws of reflection to show how a plane mirror produces a virtual image and where that is located The use of light rays also demonstrates another interesting property that allows you to locate the image more directly

Using Equal Perpendicular Lines There are two interesting observations to make about an object and its image in a plane mirror: The distance from the object to the mirror is exactly the same as the distance from the image to the mirror (the image appears to be located the same distance behind the mirror as the object is in front of the mirror) The object−image line is perpendicular to the mirror surface.

Using Equal Perpendicular Lines The use of equal perpendicular lines allows you to easily locate the image of an object All you need to do is pick several points on the object and then use the object−image lines and the mirror to locate the image. When you have enough points, you can draw the virtual image

Plane Mirrors Virtual Image Real Image Reconstructs the light off the mirror to appear as it there is something behind the mirror Interprets the light rays as if its there (behind the mirror) When all the light rays focus on a surface as light reflects off the mirror Has to focus on an actual surface Example: the Light from a projector has light rays converge when hits the screen Light rays real converge If you take a light source and project those rays from it they will meet and form a real image

The Acronym: SALT When you describe the properties of an image, you need to examine four characteristics: Size of image (compared to the object: same size, smaller, or larger) Attitude of image (which way the image is oriented compared to the object: upright or inverted) Location of image (in relation to the normal) Type of image (real or virtual) A real image is an image formed when light is actually arriving at the image location.

The Acronym: SALT An Image in a Plane Mirror is ALWAYS The same size as the object (Size), Upright but laterally inverted (left becomes right / right becomes left) (Attitude), Behind the mirror (Location: the same distance behind as the object is in front), and virtual (Type) laterally inverted (if you stand in front of a mirror, your left side seems to be on the right in the reflection)

Drawing RAY Diagrams Draw the image of the object The object distance is equal to the image distance to determine the exact location of the object. Pick one extreme on the object and carefully measure the distance from this extreme point to the mirror. Mark off the same distance on the opposite side of the mirror and mark the image of this extreme point. Repeat this process for all extremes on the object until you have determined the complete location and shape of the image.

Drawing RAY Diagrams  Pick one extreme on the image of the object and draw the reflected ray that will travel to the eye as it sights at this point The eye must sight along a line at the image of the object in order to see the image of the object Draw a bold line for the reflected ray (from the mirror to the eye) and a dashed line as an extension of this reflected ray; the dashed line extends behind the mirror to the location of the image point The reflected ray should have an arrowhead upon it to indicate the direction that the light is traveling The arrowhead should be pointing towards the eye since the light is traveling from the mirror to the eye, thus enabling the eye to see the image.

Drawing RAY Diagrams Draw the incident ray for light traveling from the corresponding extreme on the object to the mirror Since you drew the reflected ray in step 2, the point of incidence has already been determined; the point of incidence is merely the point where the line of sight intersects the mirror's surface. Thus draw the incident ray from the extreme point to the point of incidence. Be sure to draw an arrowhead upon the ray to indicate its direction of travel. The arrowhead should be pointing towards the mirror since light travels from the object to the mirror.

Drawing RAY Diagrams Repeat steps 2 and 3 for all other light rays By finishing steps 2 and 3, you have only shown how light travels from a single light ray on the object to the mirror and finally to the eye Next, we show how light travels from the other extremes on the object to the eye.

Review https://www.youtube.com/watch?v=2ek0EsEMTBc

Let’s Try Suppose that six students - Al, Bo, Cy, Di, Ed, and Fred sit in front of a plane mirror and attempt to see each other in the mirror. And suppose the exercise involves answering the following questions: Whom can Al see? Whom can Bo see? Whom can Cy see? Whom can Di see? Whom can Ed see? And whom can Fred see?

Let’s Try! So it would look like this …

Let’s Try! Locate the images of the given students. Al is isolated from the rest of the students and lines of sight are drawn to see who Al can see. The leftward-most student whom Al can see is the student whose image is to the right of the line of sight that intersects the left edge of the mirror. This would be Ed. The rightward-most student who Al can see is the student whose image is to the left of the line of sight that intersects the right edge of the mirror. This would be Fred. Al could see any student positioned between Ed and Fred by looking at any other positions along the mirror. However in this case, there are no other students between Ed and Fred; thus, Ed and Fred are the only students whom Al can see

Let’s Try! http://www.physicsclassroom.com/class/refln/Lesson-2/Ray-Diagrams-for-Plane-Mirrors

Let’s Try!

Answer

Practice Time! Complete the worksheets on plane mirrors given To be handed in beginning of next class!