1. 12.1 Refraction

2. Penny Demo

3. Magic?

5. Refraction Bending of light as it passes from one medium into another
Why?
The speed of light is:
3.00 x 108 m/s in a vacuum
2.26 x 108 m/s in a water
1.76 x 108 m/s in a acrylic

6. Rules Light bends TOWARDS to normal when the speed decreases
Light bends AWAY from normal when the speed increases (up up and away!)

7. incident ray
MEDIUM #1
MEDIUM #2

8. incident ray
MEDIUM #1
MEDIUM #2

9. incident ray
MEDIUM #1
MEDIUM #2

10. Sample Question
If I had a beam of light passing from a water medium to an acrylic medium, would the light bend towards or away from the normal?

11. Sample Question
If I had a beam of light passing from a water medium to an acrylic medium, would the light bend towards or away from the normal?

12. Angle of Refraction The angle between the refracted ray and the normal
(Add this to your diagram)

13. Partial Reflection and Refraction
Some light that hits a surface refracts into the new medium
Some of the light is reflected back into the same medium
Ex. two way mirrors and mirrored sunglasses

15. Vital Science Video

16. Homework
Pg 519 #1-7

17. 12.4 Index of Refraction (Using Speed)
The speed of light in a medium is always less than the speed of light in a vacuum.
Index of refraction (n): the ratio of the speed of light in a vacuum to the speed of light in the specified medium.
𝒏= 𝒄 𝒗
𝒘𝒉𝒆𝒓𝒆:
𝒏 𝑖𝑠 𝑡ℎ𝑒 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛
𝒄 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎 𝑣𝑎𝑐𝑢𝑢𝑚
𝒗 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑒𝑑 𝑚𝑒𝑑𝑖𝑢𝑚
Note: Both c and v are in m/s. Dividing will cause the units to cancel leaving “n” unit-less (hence it is a ratio)

18. The speed of light in sodium chloride is 1. 96 x 108 m/s
The speed of light in sodium chloride is 1.96 x 108 m/s. Calculate the index of refraction for sodium chloride.
Sample Problem

19. Sample Problem
The speed of light in sodium chloride is 1.96 x 108 m/s. Calculate the index of refraction for sodium chloride.
Givens:
c = 3.00× 10 8 m/s
v = 1.96 x 108 m/s
n =?
𝑛= 𝑐 𝑣
𝑛= 3.0× 10 8 𝑚/𝑠 1.96× 𝑚/𝑠
n= 1.53

20. Sample Problem #2
Calculate the speed of light through olive oil (n = 1.48)
Givens:
c = 3.00× 10 8 m/s
n = 1.48
v =?
𝑛= 𝑐 𝑣
1.48 = 3.00× 10 8 m/s 𝑣
𝑣= 3.00× 10 8 m/s 1.48
𝑣=2.03 × 10 8 m/s

21. Index of Refraction (Using Angles)
The ratio of the angle of incidence to the angle of Refraction will also give you the index of refraction
n = sin <i
sin <R
where
<i is the angle of incidence
<R is the angle of refraction
*****NEED A SCIENTIFIC CALCULATOR*****

22. Sample Problem
A light ray refracts at an angle of 50 degrees. The angle of incidence is 65 degrees. What is the index of refraction of the second material?
n = sin <i
sin <R
n = sin <65° =
sin < 50°
n = 1.18

23. Homework
Pg 525 #1-9

24. LAB: The Path of Light
Pg 520 in text (12.2)

25. 12.5 Total Internal Reflection

26. Slow Medium to Fast Medium
When light travels from one medium to another, some of the light is reflected and some is refracted.
When light speeds up at the boundary of two media, it bends away from the normal (water to air).

27. Critical Angle
Angle of incidence increases causing angle of refraction to reach up to 90º
critical angle: the angle of incidence that produces a refracted angle of 90º

28. Total Internal Reflection
total internal reflection: angles greater than the critical angle causing incident ray to be reflected back into same medium
DEMO

29. Summary: Two conditions for total internal reflection:
Light must be travelling slower in the first medium compared to the second
The angle of incidence must be large enough that no refraction occurs in the second medium

30. Applications:
Diamonds: very high n, therefore very small critical angle, therefore lots of total internal reflection = sparkle
Fiber Optics: light travels along glass cable

31. Triangular prisms: used in optical devices such as cameras and binoculars since they do not deteriorate like the reflective film in mirrors.

32. Retro reflectors: cut off from corner of cube- used to send light back parallel to incident light. Use on moon, bike reflectors and road signs.

33. Homework
Pg 531 #1-3, 5, 8