A couple of house rules
Be on time Switch off mobile phones Put away laptops
Being present = Participating actively Het basisvak Toegepaste Natuurwetenschappen
http://www.phys.tue.nl/nfcmr/natuur/collegenatuur.html Applied Natural Sciences
Leo Pel e‐mail: [email protected] http://tiny.cc/3NAB0 Content of the course 3NAB0 (see study guide)
17‐20 November diagnostic test!
Week 1 : 13 November Introduction, units (Ch1), Circuits (Ch25,26)
Week 2 : 20 November Heat (Ch17), Kinematics (Ch2‐3)
Week 3: 27 November Newton, Energy (Ch4‐6)
Week 4: 4 December Energy, Momentum (Ch7‐8)
7 December Intermediate assessment 18.15 – 19.00
Week 5: 11 December Rotation, Elasticity, Fluid mechanics (Ch9‐12)
Week 6: 18 December Harmonic oscillator and Waves (Ch14‐15)
Week 7: 8 January (2016) Sound (Ch16) Light (Ch33)
24 January Final assessment 09.00 – 12.00 Chapter 33 The Nature and Propagation of Light
PowerPoint® Lectures for University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman
Lectures by Wayne Anderson
Copyright © 2012 Pearson Education Inc. LEARNING GOALS
• What light rays are, and how they are related to wave fronts.
• The laws that govern the reflection and refraction of light.
• The circumstances under which light is totally reflected at an interface.
• How to make polarized light out of ordinary light.
5 Chapter 33: the nature and propagation of light
• The nature of light
• Reflection and refraction
• Total internal reflection
• Polarization
• Scattering of light
7 IN THE BEGINNING ‐ (4.5 Billion BC)
In the beginning it was dark and cold. There was no sun, no light, no earth, no solar system. Then slowly, about 4.5 billion years ago, a
swirling nebula, ‐ a huge cloud of gas and dust was formed.
8 THE SUN ‐ (4 Billion BC)
• Eventually this cloud contracted and grew into a central molten mass that became our sun. At first the sun was a molten glow. • As the core pressure increased, and the temperature rose to millions of degrees ‐ a star was born. Through the process of thermonuclear hydrogen fusion, the sun began to shine. • This was the nebular hypothesis, first proposed in 1755 by the great German philosopher, Immanuel Kant.
9 THE EARTH ‐ (4 Billion BC)
• Soon after the Sun was formed, the Earth and our other planets were formed from violent explosions and spinoffs from the process that created the Sun. • As rocks and other particles collided forming the Earth, it became molten. The surface of the Earth cooled and hardened.
10 EARLY LIFE ‐ (3 Billion BC)
Gradually oceans appeared and sunlight and water gave birth to life, eventually, intelligent life. Without light, there would be no life. Life was dependent on three things being present: a.) the basic long molecule building block, carbon, b.) water, c.) light.
11 LIGHT AND THE BIBLE
The bible
There are more than 200 references to the word 'light' in the Bible. About 75 of these occur in the new testament.
Light was the first of God's creations, according to the book of Genesis.
"And God said, let there be light, and there was light".
(Old Testament, Genesis, i,3.)
12 A long time ago ……
• Aristotle (384 ‐ 322 B.C.), an ancient Greek thinker, thought that we saw the world by sending “something” out of our eye and that reflected from the object.
• In Plato’s time (427 – 347 B.C.), the reflection of light from smooth surfaces was known. He was also a Greek.
• The ancient Greeks (about 200 A.D.) also first observed the refraction of light which occurs at the boundary of two transparent media of different refractive indices.
14 In the 17th century, two scientists had different views about the nature of light ……
Light is particles
No! Light is waves
Isaac Newton Christian Huygens 1643 - 1727 1629 - 1695
15 The speed of light
Speed of light in free space: 3x108m/s.
• Roemer’s (Danish astronomer) measurement by observing the eclipse of Jupiter’s satellite (1676) 2.1x108m/s.
• Fizeau’s (French scientist 1849) measurement: c= 3.15x108m/s
• 1873: James Clerk Maxwell predicted electromagnetic waves traveling at approximately the speed of light,
speed of light: c = 299,792,458 m/s (in vacuum)
16 Extra
17 Light as an electromagnetic wave
18 Observation of colors in light
Eye sensitivity
Source: wikimedia.org
19 Waves fronts
• Wave front is “the leading edge of a wave”
• More formal definition:
The locus (latin=place) of all adjacent points at which the phase of vibration of a physical quantity associated with the wave is the same
20 Rays
• A ray indicates the (local) direction of propagation of a wave front
• In a homogeneous isotropic medium rays are straight lines
• Ray description of light is called geometrical optics
21 Chapter 33: the nature and propagation of light
• The nature of light
• Reflection and refraction
• Total internal reflection
• Dispersion
• Polarization
• Scattering of light
24 Reflection and refraction
At an interface between two materials, a wave is partly reflected and partly refracted (transmitted) 26 Reflection
Light striking a surface may be reflected, transmitted, or absorbed. Reflected light leaves the surface at the same angle it was incident on the surface:
i r
Real Important Note: the angles are measured relative to the surface normal.
27 Reflection: specular & diffuse
• Smoothness of the interface between two media determines the nature of the reflection and refraction
• Primary study is specular reflection (mirror like)
28 Examples: specular and diffuse reflection
29 Refraction
Light travels in a straight line except when it is reflected or when it moves from one medium to another.
http://id.mind.net/~zona/mstm/physics/light/rayOptics/refraction/refraction1.html
Refraction—the “bending” of light rays when light moves from one medium to a different one—takes place because light travels with different speeds in different media.
30 The speed of light in a vacuum is c = 3x108 m/s. The index of refraction of a material is defined by c n = , v where c is the speed of light in a vacuum and v is the speed of light in the material.
The speed and wavelength of light change when it passes from one medium to another, but not the frequency, so
c v = and = . nnn
Because light never travels faster than c, n 1. For water, n = 1.33 and for glass, n 1.5.
32 Refractive index
Because light never travels faster than c, n 1. For water, n = 1.33 and for glass, n 1.5.
Example: calculate the speed of light in diamond (n = 2.42). c v = n
3×108 m/s v = 2.42
v = 1.24×108 m/s
33 ConcepTest
A group of sprinters gather at point P on a parking lot bordering a beach. They must run across the parking lot to a point Q on the beach as quickly as possible. Which path from P to Q takes the least time? You should consider the relative speeds of the sprinters on the hard surface of the parking lot and on loose sand.
1. a 2. b 3. c 4. d 5. e 6. All paths take the same amount of time.
34 ConcepTest
Fermat’s principle The path taken by light is such that the time of travel is minimum
1. a 2. b 3. c 4. d 5. e 6. All paths take the same amount of time.
35 Laws of reflection and refraction: Snell’s laws
• All rays are in one plane • Angle of reflection equals angle of incidence
Willebrord Snell 1591-1626
Snell’s “Law”, also called the law of refraction, gives the relationship between angles and indices of refraction:
naabb sinθ = n sin θ .
36 Question
A fish swims below the surface of the water at P. An observer at O sees the fish at
1. a greater depth than it really is. 2. the same depth. 3. a smaller depth than it really is.
37 A fish swims below the surface of the water at P. An observer at O sees the fish at
1. a greater depth than it really is. 2. the same depth. 3. a smaller depth than it really is. Note: The rays emerging from the water surface converge to a point above the fish.38 Broken stick
39 Magic
40 Refraction: why the sun becomes flattened at sunset
41 Refraction: why the sun becomes flattened at sunset
• Light refracts at the atmospheric interface
• Curvature of the interface causes a varying normal to the interface
• Hence the angle of refraction differs for every incident ray
• Flattening of the “bottom” side of the sun sphere
42 Magic
43 Magic
44 Chapter 33: the nature and propagation of light
• The nature of light
• Reflection and refraction
• Total internal reflection
• Dispersion
• Polarization
• Scattering of light
45 Total Internal Reflection
n1122 sinθ = n sin θ
n2 sinθ12 = sin θ n1
Suppose n2 n 1 sin θ = 2 . For larger than this, Snell’s 1,max Law cannot be satisfied! n1 This value of is called the critical angle, C. For any angle of incidence larger than C, all of the light incident at an interface is reflected, and none is transmitted. 46 n2 n1>n2 Ray incident normal to surface is not “bent.” Some is reflected, some is transmitted. 47 n2 n1>n2 Increasing angle of incidence… 48 n2 n1>n2 Increasing angle of incidence…more… 49 n2 n1>n2 Increasing angle of incidenceincidence…more…critical…more…critical angle reached… some of incident energy is reflected, some is “transmitted along the boundary layer. 50 n2 n1>n2 Light incident at any angle beyond C is totally internally reflected. 51 52 Total internal reflection: application 53 Total internal reflection also plays an important role in the design of jewelry The brilliance of diamond is due in large measure to its very high index of refraction (n = 2.417) and correspondingly small critical angle. Light entering a cut diamond is totally internal reflected from facets on its back surface, and then emerges from its from surface. TIR: Diamond cutting Want to MAXIMIZE reflection here Brilliant diamond cut must maximize light return through the top. 54 Light trapped in a fiber optic glass communications cable: total internal reflection 55 56 Reflection: Thin Film Interference (no examination) http://www.photographyblog.com/gallery/showphoto.php?photo=5545 …colors due to reflected waves with to path length differences. 57 57 58 If the waves out of phase, are If the waves If the waves in phase, they are If the waves reinforce to produce a wave of greater amplitude. to produce a wave reinforce they reinforce to produce a wave of reduced amplitude. to produce a wave they reinforce Interference—a Result of the Superposition Waves Result Interference—a Destructive Interference: Constructive Interference: Reflection: Thin Film Interference (no examination) Film; e.g. oil on water 59 Reflection: Thin Film Interference (no examination) Rays reflected off the lower surface travel a longer optical path than rays reflected off upper surface. Film; e.g. oil on water 60 Reflection: Thin Film Interference (no examination) Rays reflected off the lower surface travel a longer optical path than rays reflected off upper surface. Film; e.g. oil on water • If the optical paths differ by a multiple of , the reflected waves add. colors • If the paths cause a phase difference , reflected waves cancel out. 61 Chapter 33: the nature and propagation of light • The nature of light • Reflection and refraction • Total internal reflection • Dispersion • Polarization • Scattering of light 63 Dispersion • Dispersion is a general name for frequency dependent effects • A refractive index typically depends on the color (frequency) of the light • Hence: different colors refract at different angles 64 Refractive Index Depends on the Wavelength 65 Concept question When can you see a rainbow? 1. When the sun is behind you and low on the horizon 2. When the sun is behind you and high in the sky 3. When the sun is in front of you and low on the horizon 4. When the sun is in front of you and high in the sky 68 Concept question When can you see a rainbow? 1. When the sun is behind you and low on the horizon 2. When the sun is behind you and high in the sky 3. When the sun is in front of you and low on the horizon 4. When the sun is in front of you and high in the sky Answer: 1. Since the maximum angle between incident and reflected light is about 42 degrees, the sun needs to be low on the horizon and behind you (due to reflection) 69 70 Primary rainbow • Primary rainbow is formed by rays that refract twice and have one total internal reflection • Splitting of colors is due to dispersion effect of water for the two instances of refraction 71 Primary rainbow • Rays of different color originate from different positions • Red light is reflected at a larger angle compared to violet • Red is the “outer” color and violet is the “inner” color of the primary rainbow 72 Secondary rainbow • Secondary rainbow originates from double internal reflection • Red is now reflected at a smaller angle compared to violet • Secondary rainbow has reversed color ordering • Longer path causes fading of light 73 The full picture: two rainbows 74 Halo (no examination) -Refraction of light through ice crystals (cirrus clouds) - Circular shape – 220 from center of sun or moon note the inner ring is red 75 Chapter 33: the nature and propagation of light • The nature of light • Reflection and refraction • Total internal reflection • Dispersion • Polarization • Scattering of light 77 Polarization of mechanical waves • Transverse mechanical waves exhibit polarization with respect to the displacement • Linear polarization: there is only one direction of displacement • Polarization filter or polarizer: waves with a certain polarization can pass it 78 Polarization of light • Light consists of transverse electromagnetic waves • Polarization is defined as the direction of the electric field 79 Polarization filters • Light from the sun, light bulbs is not polarized: mixture of all polarizations • Natural light is unpolarized • In a polarization filter: • one polarized component is absorbed (dichroism) • one polarized component is passed on 80 Polarizing filter • Transmitted light contains only one polarization • Natural light has uniform intensity distribution over all polarizations • Hence, after a polarizer only half of the total intensity is left 81 82 Polarization by reflection • The intensity of the light reflected at an interface is angle and polarization dependent • At a specific angle θp the light with electric field parallel to the plane of incidence is not reflected at all: hence polarization 84 Polarization by reflection • θp is called the polarizing angle or the Brewster angle (1812: discovery by Sir David Brewster) • Angle of refraction satisfies ° • Snell’s law: sin sin • Brewster’s law of polarizing angle: tan 85 Polaroid sunglasses 86 Quiz: polarization Polaroid’s sunglasses are transparent for 1. horizontal polarization 2. left-skew polarization 3. any single type of polarization 4. vertical polarization 87 Quiz: polarization Polaroid’s sunglasses are transparent for 1. horizontal polarization 2. left-skew polarization 3. any single type of polarization 4. vertical polarization Answer: 4. Due to polarization by reflection, vertically polarized light will be reflected less than horizontally polarized light (assuming reflection interface is mostly horizontal: sea, road, etc) 88 89 Birefringent or double refracting • Birefringence materials: • Difference in refractive index in two polarization directions • One polarization travels faster through the medium than the polarization in the orthogonal direction • Common example: CaCO3 (Calcite) 90 91 Photoelasticity • Some materials become birefringent under mechanical stress, e,g, plastics • Hence mechanical stress can be visualized and studied through optical methods by exploiting birefringence and using polarization filters 92 Stress Concentration Dense fringe indicates higher stress concentration 93 2D Photo Elastic fringes : real time stress development 94 Chapter 33: the nature and propagation of light • The nature of light • Reflection and refraction • Total internal reflection • Dispersion • Polarization • Scattering of light 96 Scattering of light: why the sky is blue… • Sunsets are red… • The sky is blue… because of scattering by air molecules! • Scattering is the absorption and/or re‐radiation of light in a variety of directions re-radiated light incident light 97 Rayleigh scattering Violet light λ=380 Intensity ratio = 15 Red light λ=750 98 98 Scattering of light: why the sky is blue… • Scattering of blue and violet light is 15 times higher than for red light (air molecules) • Red light is less scattered than blue light and therefore sunsets are red: long path through the atmosphere safe to look at sunset, as harmful UV rays are removed by scattering through thick slice of atmosphere 99 Scattering of light: why the sky is blue… • Scattering of blue and violet light is 15 times higher than for red light (air molecules) • Blue light is scattered more that red light and therefore the light reflected by the atmosphere should be violet 100 But why isn’t the sky violet? • Our eyes are much more sensitive to blue than to violet • Hence the color of the sky as we see it is blue Source: wikimedia.org 101 Scattering of light: why the sky is blue and the clouds white … • Scattering of blue and violet light is 15 times higher than for red light (air molecules) • Water droplets in a cloud are much larger than air molecules & scattering from them is almost independent of wavelength – appear white 102 Green Flash (ken uw filmklassiekers) 103 104 Sunset: Green Flash (no examination) 1. as red light (long λ) from sun is just about to disappear, the blue and green light (shorter λ) is still above horizon 2. the blue light has been removed by Rayleigh scattering, leaving green light Green flash!!! 105 Green Flash (no examination) Rare condition seen at coast - refraction of different colors of light - seen a brief “flash of green” 106 107 Light mill; radiometer 108 Radiation pressure : magnitude of the momentum change Light = Photons: particles which have a momentum (particle) incident 2I Prad = (total reflection) c reflected Very small!!! Solar sail 109 Light—Waves or Particles? http://www.nearingzero.net (quantum007.jpg) Both: You need Quantum mechanics to explain all 110 110 Good luck with your examination 114