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• Here can do cool light with ½ silvered mirror demo (1041)
• Bottom of mirror must be lower than halfway between eyes and feet to see feet. Demo 1095, ask for volunteer
• Demo 435
• demo: 1131
• still true that angle of incidence=angle of reflection
• demo: 44 (light the match)
• virtual image
• ### Lect16 handout

1. 1. Introduction to Mirrors Physics 102: Lecture 16
2. 2. Phys 102 recent lectures <ul><li>Lecture 14 – EM waves </li></ul><ul><li>Lecture 15 – Polarization </li></ul><ul><li>Lecture 20 & 21 – Interference & diffraction (coming soon!) </li></ul><ul><li>Lecture 16 – Reflection </li></ul><ul><li>Lecture 17 – Spherical mirrors & refraction </li></ul><ul><li>Lecture 18 – Refraction & lenses </li></ul><ul><li>Lecture 19 – Lenses & your eye </li></ul>Light as a wave Light as a ray
3. 3. Light incident on an object <ul><li>Absorption </li></ul>Everything true for wavelengths << object size <ul><li>Reflection (bounces) </li></ul><ul><ul><li>See it </li></ul></ul><ul><ul><li>Mirrors </li></ul></ul><ul><li>Refraction (bends) </li></ul><ul><ul><li>Lenses </li></ul></ul><ul><li>Often some of each </li></ul>
4. 4. Recall: wavefronts Wavefronts represent “crests” of EM wave x z Top view: Direction of propagation x z y y z y z  
5. 5. Huygens’ principle Every point on a wavefront acts as a source of tiny “wavelets” that move forward. “ wavelet” wavefront Direction of propagation
6. 6. Law of Reflection Incident wave Reflected wave Describe light as “rays” along direction of propagation of EM wave  i  r  i =  r Angle of incidence = Angle of reflection
7. 7. Object Location <ul><li>Light rays from sun bounce off object and go in all directions </li></ul><ul><ul><li>Some hit your eyes </li></ul></ul>We know object’s location by where rays come from. <ul><li>Color results from some light being absorbed by object before bouncing off. </li></ul>
8. 8. Flat Mirror <ul><li>All you see is what reaches your eyes </li></ul><ul><ul><li>You think object’s location is where rays appear to come from. </li></ul></ul>Smooth Mirror Object Image All rays originating from peak will appear to come from same point behind mirror!  r  i
9. 9. Flat Mirror <ul><li>(3) Lines appear to intersect a distance d behind mirror. This is the image location. </li></ul>Virtual: No light actually gets here d (1) Draw first ray perpendicular to mirror 0 =  i =  r (2) Draw second ray at angle.  i =  r Light rays don’t really converge there, so it’s a “Virtual Image” Example d  r  i
10. 10. Flat Mirror Summary <ul><li>Image appears: </li></ul><ul><ul><li>Upright </li></ul></ul><ul><ul><li>Same size </li></ul></ul><ul><ul><li>Located same distance from, but behind, mirror </li></ul></ul><ul><ul><li>Facing opposite direction: Left/Right inverted </li></ul></ul><ul><ul><li>Virtual Image: Light rays don’t actually intersect at image location. </li></ul></ul>Preflight 16.1 Why do ambulances have “AMBULANCE” written backwards? So you can read it in your rear-view mirror!
11. 11. Preflight 16.3 Can you see Fido’s tail in mirror? (You) (Fido) mirror
12. 12. ACT: Flat Mirrors <ul><li>You are standing in front of a short flat mirror which is placed too high, so you can see above your head, but only down to your knees. To see your shoes, you must move </li></ul><ul><ul><li>(1) closer to the mirror. </li></ul></ul><ul><ul><li>(2) further from the mirror. </li></ul></ul><ul><ul><li>(3) to another mirror. </li></ul></ul>
13. 13. ACT: Flat Mirrors to see feet, bottom of mirror must extend at least as low as midpoint between eyes and feet—independent on how far you are from the mirror.
14. 14. ACT/Two Mirrors <ul><li>How many images of money will you see (not including the actual money)? </li></ul><ul><li>1 </li></ul><ul><li>2 </li></ul><ul><li>3 </li></ul><ul><li>4 </li></ul><ul><li>5 </li></ul>
15. 15. Curved mirrors A Spherical Mirror: section of a sphere. C = Center of curvature In front of concave mirror, behind convex mirror. R principal axis light ray • Concave mirror R C Convex mirror principal axis light ray R C •
16. 16. Preflight 16.2 An organic chemistry student accidentally drops a glass marble into a silver nitrate mirroring solution, making the outside of the marble reflective. What kind of mirror is this? (1) concave (2) convex (3) flat
17. 17. Concave Mirror Principal Axis Rays parallel to principal axis and near the principal axis (“paraxial rays”) all reflect so they pass through the “Focus” (F). R The distance from F to the center of the mirror is called the “Focal Length” (f). Angle of incidence = angle of reflection. Thus rays are bent towards the principal axis. Focus f=R/2
18. 18. Preflight 16.4, 16.5 What kind of spherical mirror can be used to start a fire? concave convex How far from the match to be ignited should the mirror be held? farther than the focal length closer than the focal length at the focal length
19. 19. Concave Mirror Principal Axis Rays traveling through focus before hitting mirror are reflected parallel to Principal Axis . Rays traveling parallel to Principal Axis before hitting mirror are reflected through focus F F
20. 20. Convex Mirror Principal Axis Focus Rays parallel to principal axis and near the principal axis (“paraxial rays”) all reflect so they appear to originate from the “Focus” (F). R f=-R/2 The distance from F to the center of the mirror is called the “Focal Length” (f). Rays are bent away from the principal axis.
21. 21. <ul><li>A concave mirror has a positive focal length f > 0 </li></ul><ul><li>A convex mirror has a negative focal length f < 0 </li></ul><ul><li>What is the focal length of a flat mirror? </li></ul><ul><li>(1) f =0 (2) f = ∞ </li></ul>ACT: Mirror Focal Lengths