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Chapter 33 the Nature and Propagation of Light 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 • Dispersion • 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 31 Refractive index 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<n1. The largest possible value of sin(2) is 1 (when 2 = 90). The largest possible value of sin(1) is 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.
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