LECTURE 11 REFLECTION & PLANE MIRRORS Instructor: Kazumi Tolich
Lecture 11 2 Reading chapter 26.1 to 26.2 Wave fronts and rays The law of reflection Plane mirrors
Wave fronts 3 The wave crests from spherical waves in 3D are called wave fronts. The radial motion of the waves is indicated by rays that are always perpendicular to wave fronts. The curvature of the wave front becomes small as you move away from the source, and the wave then becomes a plane wave.
The law of reflection/demo: 1 4 The angle between the incident ray and the normal is called the angle of incidence, θ ". The angle between the reflected ray and the normal is called the angle of reflection, θ #. The incident and reflected rays and the normal lie on the same plane, and the law of reflection states: θ " = θ # Demo: angle of incidence and reflection
Quiz: 1 5 When watching the Moon over the ocean (VERY romantic), you often see a long streak of light on the surface of the water. This occurs because A. the Moon is very large. B. atmospheric conditions are just right. C. the ocean is calm. D. the ocean is wavy. E. motion of the Moon.
Quiz: 11-1 answer 6 the ocean is wavy. When the water surface changes, the angle of incidence also changes. Thus, different spots on the water can reflect the Moon into your eyes at different times.
Specular and diffuse reflections/demo: 2 7 Reflection from a smooth surface is called specular reflection; if the surface is rough, it is diffuse reflection. Light reflected off a dry street is a diffuse reflection, but light reflected off a wet street becomes more specular, and it makes difficult to see what is on the road. Demo: Diffuse/specular reflection
Plane mirrors 8 Light from an object is reflected off the mirror obeying the law of reflection and enters the eye. The eye interprets the ray as having had a straightline path, and sees the image behind the mirror. Properties of mirror images produced by plane mirrors: Image is upright but appears reversed right to left. Image appears to be the same distance behind the mirror that the object is in front of the mirror. Image is the same size as the object.
Left/right reflection?/demo: 3 9 Why does a mirror reverse left and right when it does not reverse up and down? A mirror reverses neither left and right nor up and down. It reverses front and back. This has the effect of making a left hand into a right hand, and vice versa. This is called depth inversion.
Quiz: 2 10 An observer at point O is facing a mirror and observes a light source S. Where does the observer perceive the mirror image of the source to be located? O S mirror
Quiz: 11-2 answer 11 Trace the light rays from the object to the mirror to the eye. Since the brain assumes that light travels in a straight line, simply extend the rays back behind the mirror to locate the image. O S mirror
Demo: 4 12 Location of Image (candle in glass of water) Demonstration of image point.
Quiz: 3 13 You stand in front of a mirror that is just tall enough for you to see your whole body in the mirror. If you get closer to the mirror, what do you see in the mirror? Choose all that apply. A. You no longer see all of your head. B. You no longer see all of your feet. C. You see your head and space above your head. D. You see your feet and space below your feet. E. You see exactly the same: just your whole body.
Quiz: 13-3 answer 14 You see exactly the same: just your whole body. The closer you step forward, the larger the incident and reflected angles will be. But the rays will still be reflected at the same points, so the ray from the foot will still be reflected at midheight.
Quiz: 4 15 At a hair salon, you are trying to see back of your head. You hold a hand mirror 0.5 m in front of you and look at your reflection in a full-length mirror 1.0 m behind you. How far in back of the hand mirror do you see the closest image of back of your head, in meters? 1.0 m 0.5 m
Quiz: 11-4 answer 16 2.5 m The image of the head reflected in the big mirror appears 1.0 m behind the big mirror. This image (which is the object for the small mirror) is 2.5 m away from the hand mirror. The final image is 2.5 m behind the hand mirror. 1.0 m 1.0 m 0.5 m Big mirror small mirror Image of head in big mirror Head Image of head in small mirror 2.5 m 2.5 m
Example: 1 (Walker Ch. 26-15) 17 You hold a small plane mirror, x &' = 0.50 m in front of your eyes. The mirror is h = 0.32 m high, and in it you see the image of a tall building behind you. A. If the building is x )* = 95 m behind you, what vertical height of the building, H, can be seen in the mirror at any one time? B. If you move the mirror closer to your eyes, does your answer to part A. increase, decrease, or stay the same?
Corner reflectors/demo: 5 18 A corner reflector reflects light parallel to the incident ray, no matter the incident angle. This is used for bicycle reflectors etc.