Ch. 35 – Reflection of Light

1. Ch. 35 – Reflection of Light What do mirrors do?

2. Cookie! What does it mean to “see” something? • Usually light from a light source reflects off the object and goes toward your eyes

3. Fermat’s Principle of Least Time Light travels in straight lines and will take the quickest path to get from point A to point B Wrong Path TruePath MIRROR

4. normal incident ray object distance reflected ray angle of incidence qi angle of reflection qr image distance The Law of Reflection “object” look into mirror look directly at lightbulb, or… “observer” mirror Law of Reflectionqi = qr “image” Brain will extrapolate backwards, where the reflected ray(s) appears to have originated from

5. That’s what this bumper sticker means Light rays are reversible reflected ray incident ray reflected ray incident ray Homer can also see Bart along the path shown Bart can see Homer along the same path mirror

6. It’s simple geometry! At what angle will light reflect off Mirror B? Mirror B Mirror A Light ray

7. 60° angle of reflection Example 2 QUESTION What will be the angle of reflection off Mirror B? Normal line 110° A 40° B

8. 80º Example 3 QUESTION A light ray is incident on a mirror at 30º. What is the angle of reflection toward the person shown? 140º 30º

9. Example 4 QUESTION What is the angle of reflection off M2?

10. Can’t shoot pool? • Your brain then “creates” this path from your eye to where it thinks the light ray originally came from What is this picture trying to say? • One of the light rays which reflects off the red ball follows this path to the mirror • The light ray then follows this path to your eye (at the cue ball)

11. Specular Reflection Specular reflection is “mirror-like” reflection If you shine the light like this, who will see the light bulb’s reflection? Where is the image? only Maggie Light only reflects in one direction

12. Diffuse Reflection Though a sheet of paper appears smooth, look more closely. The surface is not “mirror-like” Notice that light still obeys the law of reflection Light reflects in all directions Close-up magnification of a surface

13. Specular vs. Diffuse Reflection Regardless of whether it is specular or diffuse, light always obeys the law of reflection. No exceptions Diffuse reflection Specular reflection

14. Wednesday, April 30

15. “AMBULANCE”

16. homework QUESTION A light ray is incident on a mirror at 30º. What is the angle of reflection toward the person shown? 30º 70º

17. 130º 30º 70º homework

18. Only Curly can see the reflection in the mirror No mirror here mirror object Larry Curly Moe

19. There can only be one image (if there is one mirror) Everyone sees the image in the same place, regardless of where you’re standing image mirror object Larry Curly Moe

20. Pepper’s GhostA clever application of reflection: • Devised in 1862 • A ghost could interact with stage members • Used in productions for Macbeth, Hamlet, and A Christmas Carol

21. How does Pepper’s Ghost work? • By careful manipulation of the lighting, the audience could see actors on the stage through the glass, as well as images of the ghost actor who is below the stage glass real actor “ghost” observer actor portraying ghost

22. John Henry Pepper (1821 – 1900) • Taught chemistry at London’s Royal Polytechnic Institution • Devised the illusion which became known as Pepper’s Ghost in 1862 • Variations of Pepper’s Ghost are still used to this day, using: • different types of reflecting surfaces (glass, mirrors, tinted glass, etc.) and • different lighting techniques

23. A modern Pepper’s Ghost Study this diagram • How is the ghost image seen playing the piano? • Where does the observer stand? From Phantasmechanics (http://www.phantasmechanics.com/pepper.html)

24. Disneyland’s Haunted Mansion Another variation of Pepper’s Ghost

25. Cockpit displays use Pepper’s Ghost • A non-entertainment variation of Pepper’s Ghost is found in jet planes • Data is superimposed onto a clear plate at eye-level • Pilots can see important information without looking away

26. A better view of the “HUD” The “heads-up display,” or HUD is visible (as the greenish plate).

27. A virtual image off a lake surface Your brain “thinks” this is where the light rays came from Note that light from different parts of the tree reflect off different parts of the lake surface

28. Echo and Narcissus (1903)John William Waterhouse

29. Good or Bad Physics? • Have a friend look into a mirror, as shown in this cartoon • Now, you stand off to the side, and look at the mirror • What do you see?

30. How much of your face can you see in a mirror? • Hold mirror a few inches in front of your face • Align forehead with top edge of mirror • Align chin with bottom edge • Move back and forth • Draw a ray diagram which explains what you observe

31. 10 feet homework How far away does the flower appear, when she looks straight ahead?

32. homework • What is the smallest mirror that enables you to see your entire body? • Where does bottom edge of mirror need to be?

34. Light rays can sometimes be bent without passing from one substance to another • In air, this happens when light travels through layers that are at different temperatures • Cold air is more dense than warm air, so it acts like a different substance • The result can be spectacular, as this old engraving shows

36. Thursday, April 19

37. Question How does this mirror work? From a Niemann Marcus Christmas catalog

38. Homework Question Draw how the mirrors must be positioned in a periscope. Label qi and qr

39. Question Let's to turn to some plain old, mass-audience common sense to come up with our own 'findings' (and without the academic baffle-gab). This is exemplified in a scene from the first Harry Potter movie, the one where Harry stumbles on the "Mirror of ERISED." (DESIRE backwards) The eleven-year-old Harry looks into the mirror and, instead of his own reflected image, sees his long-dead parents, who he has been yearning all his life to meet. Harry's friend looks into the mirror and sees himself as a successful and celebrated athlete. It seems that what this peculiar mirror reflects are the deepest desires of the looker-in. When the headmaster finds Harry in front of the mirror, he warns him that people have wasted away in front of the glass, pining for what they desire, rather than getting on with their lives. He also tells Harry that a person who was content with his life would see only himself reflected in the mirror.