1. Nature of Light
Chapter 13

2. What is light? Electromagnetic radiation (needs no medium)
Visible light is the electromagnetic radiation our eyes can detect
700nm to 400 nm wavelengths
ROYGBIV from 700nm-400nm

3. Wave particle duality Light acts like a wave (interferes)
Light also acts like a particle – distinct packet of energy - a photon
This is a part of physics still being researched

4. Ray model of light Ray model – light travels in straight paths
Very helpful in examining reflection and refraction

5. Speed of Light
Until 17th century – all thought light arrived instantaneously
From Ole Roemer measured Io’s time behind Jupiter – found it took 22 minutes for light to cross the diameter of earth’s orbit
This value was a little low

6. Speed of Light II
Albert Michelson used earth-based techniques, the distance between 2 mountains 35 km apart
In 1926 He got X 108 – very close to the value we use today - he won the Nobel prize for this work

7. Speed of light today
The speed of light (in a vacuum) is an important universal constant
Its symbol is c and its value is 299,792,458 m/s
For our calculations use c = 3.00 X 108 m/s

8. Calculations Still a wave: v = λf
Since light has a constant velocity (in vacuum) c = λf
For light – higher frequency = higher energy
EM Spectrum

9. Materials and light
Transparent – lets light through without distortion (windows, clear glass)
Translucent – lets light through but can’t clearly see objects (stained glass, frosted lightbulbs)
Opaque – does not let light through (wood, brick, steel)

10. Reflection
Law of reflection – a ray of light will reflect at an angle equal to the angle of incidence with respect to the normal Θinitial = Θreflected

11. Mirror Diagrams Use Ray diagrams to examine Mirror problems
Flat mirrors
Telescopes

12. Regular vs. Diffuse reflections

13. Refraction Light at transmission boundary
Light is bent, depending on how optically dense the new material is
Angle of refraction is angle with normal (perpendicular) in new material

14. Refraction II
From less optically dense to more dense -> angle of refraction is smaller than angle of incidence
From more dense to less dense -> angle of refraction is larger than angle of incidence
Car on mud and road

15. Snell’s Law
Angles are related – the ratio of sin Θi to sin Θr is related to the density of the 2 media
(sin Θi)/(sin Θr)= nr/ni or ni sin Θi = nr sin Θr
Where n is the index of refraction for the medium, n = (sin Θi)/(sin Θr) where light is passing from a vacuum into the medium

16. Snell’s Law example
Light enters water (n = 1.33) from air (n = 1.00) at an angle of 35°. At what angle is the light refracted?
ni sin Θi = nr sin Θr
(1) sin(35) = (1.33) sin(x)
Sin-1[(1)sin(35)/(1.33)] = x = 25.5°

17. Snell’s Law example 2
Light enters crown glass (n = 1.52) from water (n = 1.33) at an angle of 55°. At what angle is the light refracted?
ni sin Θi = nr sin Θr
(1.33) sin(55) = (1.52) sin(x)
Sin-1[(1.33)sin(55)/(1.52)] = x = 45.8°

18. Uses of Refraction Lenses – focus or spread light
Convergent
Divergent
Eyeglasses – near or far sighted
Hologram

19. Eyeglasses and how they work

20. How to use a lens focus effectively
1/di + 1/do = 1/F

21. Speed of light in substances
Revisit pavement and mud
Vehicle turns because it moves slower in mud, faster on pavement
Remember from Ch 12 that speed changes at boundaries and depends on the material
Light is slower in materials than in vacuum

22. Speed of light in substances II
How much slower is determined same way as how much the light bends, they are related
Stickier mud would turn you sharper, slow you down more
In light we use the index of refraction (n)
vsubstance = c/nsubstance

23. Light speed problem
Light enters water (n = 1.33). What is the speed of the light in the water?
vsubstance = c/nsubstance
vsubstance = [(3.00*108 m/s)/1.33]
vsubstance = 2.26*108 m/s

24. Color from white light Prisms/rainbows – color from white light
White light is made up of colored light – Light Box
Light mixing website
Red + Green + Blue = White
These are the primary colors of addition

25. Light by addition Secondary colors – from pairs of primary colors
Yellow, Cyan, Magenta are the secondary colors of addition
Each 2nd color is a complementary color to the primary color that was removed to make it
Ex: Blue + Red = Magenta, so Magenta is complement to Green

26. Colors by subtraction
We see objects as colors because that is the wavelength (or combination of wavelengths) that it reflects, absorbing the rest
Black objects absorb all colors, white reflects all colors
Dyes are molecules that absorb certain wavelengths and reflect the rest Pigments are whole materials that do this

27. Colors by subtraction II
A primary pigment is one that absorbs 1 primary color (thus it is the mix of the other 2)
They are the same as the secondary (complementary) colors – Yellow, Cyan, Magenta
Secondary pigments absorb 2 primary colors, therefore they are the primary colors (one is left)

28. Color summary Addition vs. Subtraction - Flag
Primary colors = secondary pigments = Red, Blue, Green
Secondary colors = complementary colors = primary pigments = Yellow, Cyan, Magenta

29. Polarized sunglasses Polarized sunglasses do what?
How do they reduce glare?
They use the fact that light can be polarized
3-D movies

30. Eyes Blind spot in the eye Rods – Dim light receptors
Cones – Color receptors
3 types, if one is not working - colorblindness

31. Eyes

32. Why… Is the sky blue? Are clouds white?
Molecules in the air scatter light
The tinier the particle, the higher the frequency of light that will scatter
Our eyes are not sensitive to violet
Are clouds white?
Droplets that compose a cloud come in a variety of sizes
This causes a variety of colors to be scattered

33. Why…
Are sunsets Red?
Low frequencies of light are scattered the least by nitrogen and oxygen molecules
Red, Orange, and Yellow light are transmitted through atmosphere more readily than violet and blue
Red light passes through the most atmosphere without interacting with matter