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, �". ¨ The angle between the reflected ray and the normal is called the angle of reflection, �#. ¨ The incident and reflected rays and the normal lie on the same plane, and the law of reflection states: �" = �# ¨ Demo: angle of incidence and reflection Quiz: 1 5 Specular and diffuse reflections/Demo: 2 6 ¨ 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 7 ¨ 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 straight- line 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 8 ¨ 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 9 Demo: 4 10 ¨ Location of Image (candle in glass of water) ¤ Demonstration of image point. Quiz: 3 & 4 11 Example: 1 (Walker Ch. 26-15) 12 ¨ You hold a small plane mirror, �&' = 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 �)* = 95 m behind you, what vertical height of the building, �, 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 13 ¨ A corner reflector reflects light parallel to the incident ray, no matter the incident angle. ¨ This is used for bicycle reflectors etc..