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Measuring the speed of light: Galileo Galileo’s assistant 1667 And God said, Let there be light: and there was light. ~ Genesis 1:3 I was unable to make sure whether the facing light appeared instantaneously. But if not instantaneous, light is very swift. Galileo PS 110A Hatch Ch. 11 - 2 Measuring the speed of light: Roemer (1676) 2 x 108 m/sec Io makes one complete orbit around Jupiter every 1.76 days. During the year it sometimes appeared ahead of “schedule” and sometimes behind. Must be the time to travel farther. PS 110A Hatch Ch. 11 - 3 Measuring the speed of light: Armand Fizeau (1849) 3 x 108 m/sec PS 110A Hatch Ch. 11 - 4 So what is light? Newton thought light was particles because it cast sharp shadows This ( ~) speed accepted today using modern techniques (short pulse propagation time, e.g., laser techniques). PS 110A Hatch Ch. 11 - 5 Particles reflect and refract (as do waves) when certain assumptions are made about an interface A wave would diffract and produce “soft edges” whether or not you have a pinhole. PS 110A Hatch Ch. 11 - 6 PS 110A Hatch Ch. 11 - 7 What happens when waves strike multiple slits? Constructive Interference Destructive Interference What happens when particles strike slits? Single slit scatter pattern Double slit scatter pattern PS 110A Hatch Ch. 11 - 8 PS 110A Hatch Ch. 11 - 9 Thomas Young -diffraction Diffraction: let’s experiment! He showed that light had wave properties, it just has a very short wavelength Light exhibits single slit diffraction single-slit diffraction (fingers) diffraction of a laser beam (Demo) P1: What happens to the light spread as (~1801) the slit gets smaller, i.e., closer to the wavelength of light. PS 110A Hatch Ch. 11 - 10 Thomas YoungYoung-interference PS 110A Hatch Ch. 11 - 11 What will particles do? Pattern when you change slit width? P2: Pattern when you change slit separation distance? Light exhibits interference PS 110A Hatch Ch. 11 - 12 PS 110A Hatch Ch. 11 - 13 So light has wave properties. What is waving? Interference: laser beam A “field” associates attributes with points in space. Now light through two slits! A temperature field: Young’s double-slit (Demo) effect of changing slit separation (and width) Hot Warm Cold Demo: two waves model difference PS 110A Hatch Ch. 11 - 14 Electric and Magnetic fields PS 110A Hatch Ch. 11 - 15 Light as an electromagnetic wave + E.&M. fields tell how a magnet points or charged particle responds No need for a propagation medium! PS 110A Hatch Ch. 11 - 16 Accelerating Electrons PS 110A Hatch Ch. 11 - 17 Color Newton saw the rainbow created by a prism. He wondered “was the color in the glass or in the light”? Electromagnetic radiation is given off whenever electrons accelerate. It, in turn, causes electrons elsewhere to accelerate. (TV, microwave oven) PS 110A Hatch Ch. 11 - 18 PS 110A Hatch Ch. 11 - 19 Color and frequency (wavelength) of light The electromagnetic spectrum Newton demonstrated that the colors were in the light and not in the prism. Thomas Young proved that color was associated with wavelength. Same speed, different frequency u u u PS 110A Hatch Ch. 11 - 20 What we knew about light in 1900 PS 110A Hatch Ch. 11 - 21 Maxwell (1864) Light behaves as an electromagnetic wave Light diffracts, refracts, interferes, and reflects The frequency of light waves is perceived as color Blue light diffracts less than red light. PS 110A Hatch Ch. 11 - 22 What can we conclude? the equations that showed that the electric and magnetic fields could “wave” wave” & predicts speed of light PS 110A Hatch Ch. 11 - 23 Light emission Planck ~ 1900 Objects glow with a characteristic color depending on temperature. The emission spectrum doesn’ doesn’t match what would be expected using classical electromagnetic waves, energy must be a multiple of an elementary unit E = h ν Light has wave nature P3: What type of wave is it? What is the medium needed to travel in? Demo: polaroid sheets PS 110A Hatch Ch. 11 - 24 PS 110A Hatch Ch. 11 - 25 Does light have a “particle” nature? The photoelectric effect If so, it will look “lumpy” Wave absorption should be continuous Particle absorption should be as a packet Shine light on metal surface and electrons are given off. Energy in a normal wave is proportional to amplitude, not frequency. i.e. will a little or big wave knock you over at the beach -does brighter light mean more energy in electrons? PS 110A Hatch Ch. 11 - 26 photoelectric effect - what do we find? PS 110A Hatch Ch. 11 - 27 Photoelectric effect The effect cannot be explained by thinking light behaves as a wave!!! Let’s experiment with light! visible light brighter visible light longer time dim UV light BUT The effect can be explained by thinking light behaves as a stream of particles!!! PS 110A Hatch Ch. 11 - 28 The Photoelectric Effect PS 110A Hatch Ch. 11 - 29 Why the “big deal”? It can only be explained if light is interacting like a particle with the electrons in the metal! Each individual photon has an energy of hf where h = Planck’s constant (very small) and f = frequency. E = hf =W + kinetic energy of electron Consider making a print with uv light PS 110A Hatch Ch. 11 - 30 Photoelectric effect shows that our classical (usual) physics doesn’t work. How can a wave make individual spots? PS 110A Hatch Ch. 11 - 31 Photon Wave Particle Duality Light is both a wave and a particle. • it is a particle of light It behaves like a wave when unobserved – Energy – No mass – No charge It travels through both slits like a wave It is detected like a particle It hits the screen as individual dots • number of photons makes up the brightness (amplitude) of light beam PS 110A Hatch Ch. 11 - 32 If this bothers you, you are in good company! “All these 50 years of pondering have not brought me any closer to answering the question, ‘what are light quanta?’ These days every Tom, Dick, and Harry thinks he knows it, but he is mistaken.” ~ A. Einstein PS 110A Hatch Ch. 11 - 33 Photos: let’s experiment P4: What would happen if we shined a very dim light through the slits in Young's double-slit experiment and just detected the light striking the screen one photon at a time? PS 110A Hatch Ch. 11 - 34 PS 110A Hatch Ch. 11 - 35 Thought Experiment wave-particle duality P5: What will I see in the double slit Light is not just a wave nor just a particle, light is light: something which exhibits characteristics of both. experiment if I put some sort of detector at one of the slits that allows me to see which slit each photon goes through? Why? But…how can a stream of photons interfere? PS 110A Hatch Ch. 11 - 36 PS 110A Hatch Ch. 11 - 37 Wave Particle Duality Light is both a wave and a particle. It behaves like a wave when unobserved It travels through both slits like a wave It is detected like a particle It hits the screen as individual dots The wave nature contains a probability of the photon (particle) striking at a certain position. PS 110A Hatch Ch. 11 - 38