The Art of Bending Light

1. The Art of Bending Light It’s what we see…

2. Refraction • the bending of light when passing through a new (transparent) substance

3. Bending Light: Vocabulary • Incident Ray • Angle of Incidence • Normal • Refracted Ray • Angle of Refraction

4. Optical Density • How difficult it is for light to movethrough a substance . • High optical density  light travels more slowly • Low optical density  light travels more quickly

5. Optical Density • How difficult it is for light to movethrough a substance . • High optical density  light travels through more slowly • Low optical density  light travels through more quickly

6. Low to High Optical Density Where is the refracted angle? FST: Fast to SlowTowardsthe normal

7. Low to much higher Optical Density Where is the refracted angle? FST: Fast to SlowTowardsthe normal

8. High to lower Optical Density Where is the refracted angle? SFA: Slow to FastAwayfromthe normal

9. Quick ways to remember… • FST = Fast to Slow, Towards Normal • FAST • FST = Freaky science teachers? • SFA = Slow to Fast, Away From Normal • SFA = Sarah Farts a lot?

10. Page 54

11. Draw the path light takes from the penny to your eye. • Where will the light bend? • Which substance has a higher optical density? • What direction do you expect light to bend?

12. Why? • Imagine rows of hikers • From dry land to swamp • Hikers in swamp mud slow down • Others stay fast • Result: Change in Angle!

13. Light Traveling from a Fast to a Slow Medium will bend _______ the normal towards

14. OR… • Pierre de Fermat • Fermat’s Principle • Light always takes fastest path • So…

15. Fastest Path • Is a straight line always the fastest? • Where is light traveling the fastest? fast slow

16. Fastest Path • Is a straight line always the fastest? • Dashed line is in slow substance for less time and in faster substance for more time. • Bent path is faster overall. d1<d2 d2 fast d1 slow

17. Speeds of Light • As light travels though different substances its speed changes

18. Optical Density • Ratio of old to new speed • n =optical density • Also known as index of refraction

19. Optical Density • c =speed of light in a vacuum • (3.00 x 108 m/s ) •  = speed of light in substance 1. In what substance will light travel the fastest? 2. What number will n always be larger than?

20. Speed of light • n = 1 in vacuum • Fastest in space. So… • n≥1 always!

21. Index of Refractions • Table of indices of refraction for various substances

22. Which substance has greatest n? Lowest?

23. Working with Air • In a vacuum, • n = 1 • In air, • n = 1.0003 • The difference is so small that air is treated like a vacuum

24. Trigonometry “Review” • SOHCAHTOA • sin  = IN OS AN

25. Snell’s Law n1= index of refraction 1 1 = incident angle n2= index of refraction 2 2 = refracted angle n1 1 2 n2

26. 1 2 How to choose the Equation ex) How fast is light in Water? • To find Speeds or optical density • To find angles or optical density

27. How fast is light in water? • n = 1.33 • c = 3.00 x 108m/s • v = ?

28. Example 2A ray of light passes from water to quartz (n = 1.54). The angle of incidence in the water is 35°. What is the angle of refraction? Water n = 1.33 Given: 1 2 Quartz n = 1.54 Unknown:

29. Example 2A ray of light passes from water to quartz (n = 1.54). The angle of incidence in the water is 35°. What is the angle of refraction? Given: Unknown:

30. Refraction Activity! Go to pg. 64

31. Example 3Wesson oil was poured into the larger beaker until it flowed about half way up the smaller beaker. Why did the bottom of the beaker disappear? • Invisible: Light rays must be unchanged • Light does not bend between beakers • n1n2

32. Example 3Wesson oil was poured into the larger beaker until it flowed about half way up the smaller beaker. Why did the bottom of the beaker disappear? oil n = 1.47 1 pyrex n = 1.47 2

33. Example 4The speed of light in a certain substance is 1.90 x 108m/s. If light passes from air into the substance at an angle of incidence of 54.0°, what will be the angle of refraction? Given: Unknown: Find n first!

34. Example 4 Given: Want:

35. Snell’s Law Song • Singin’ n1 sine theta-sub-1, hey-hey-hey, Equals n2 sine theta-sub-2, hip hooray

36. Snell’s Law Song • Singin’ n1 sine theta-sub-1, hey-hey-hey, Equals n2 sine theta-sub-2, hip hooray

37. Start Reading Log pg 70

38. Colors in the sky

39. Extra Long Sunsets

40. Why do stars twinkle?

41. Mirages

43. Total Internal Reflection

44. Total Internal Reflection • Optical fibers (Internet) • Rain sensors to control automatic windscreen/windshield wipers. • Binoculars • Multi-touch screens (IPAD!) • Fingerprinting devices • Double Rainbows • Eye doctors, to measure the angle between the eye's cornea and iris.

45. How far can light bend? No more refraction! Light must now reflect ?

46. The i that causes r = 90° is called the critical angle

47. Sometimes light Reflects • Only possible when light travels from high n to lowbecause light needs to bend away from the normal • Occurs when incident angle > critical angle • i> icritical

48. Critical Angle Behavior air air air air glass glass glass glass

49. Calculating Critical Angle Use Snell’s Law or

50. Example What is the critical angle for light going from glass to air?(nglass= 1.50) Given: Unknown: