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Announcements • No Office hours today for DR • Exam next Thur. Mar 13 will cover all material through HW 7 • Prof. Smalyukh will lecture Fri and Mon. Today • Finish quantum nature of light • Atoms and atomic spectra Photon about to go through the slits Photon as little segment of wave moving towards slits Intensity of wave in various places, indicates probability of finding the photon there if you looked at that moment. Photon after it went through the slits Still only one photon! (But this photon has a slightly complicated wave function.) Photon is a wave… It can interfere with itself. Intensity of wave in various places indicates the probability of finding the photon at that spot, if I had detector there (e.g. a bunch of atoms or a sheet of metal) Photon after it went through the slits When photon interacts with one electron or atom, all energy ends up in (“collapses into”) one spot… Here it behaves like a particle with energy = hc/ λ Probability of hitting at a specific place is proportional to the square of the field amplitude at that location. Which slit did this photon go through? a. left b. right c. both d. neither e. either left or right we just cannot know which one Which slit did this photon go through? If one slit: Get single slit pattern (i.e. no interference) Like this: or this: The sum of the two: But not like this: But: that photon is part of the two slit interference pattern. The probability pattern of where it lands is described by the 2 slit interference pattern (the photon has to ‘know’ about both slits!) à It must have gone through both slits, i. e. as a wave! (When it interacts with the screen it behaves particle-like!) quantum-wave-interference_en.jar Random nature of the photons is a consequence of: a) quantum theory of light b) can be proven by thought experiments starting from Maxwell’s equations c) not a consequence of anything: this is just what the experiments show. Random nature of the photons is a consequence of: a) quantum theory of light b) can be proven by thought experiments starting from Maxwell’s equations c) not a consequence of anything: this is just what the experiments show. It is a postulate of quantum theory of light, i.e. a statement without theoretical explanation. This is simply what experiments show. (analogous to the speed of light being the same in all ref. frames) Atoms and atomic spectra Atoms and Atomic spectra • What are atoms made off? JELL–O? • What happens when we hit atoms with various stuff? • How do atoms interact with light? à Develop model of how light interacts with and is produced by individual atoms (Will help us learn how atoms ‘work’) Early ideas about atoms invisible! • Atom - Greek “indivisible unit” • The Blueberry-Muffin-Atom-Model (aka. ‘The Thomson model’): Uniform distribution of positive charges with negative electrons embedded in it. à But wait! We can get electrons from them (scraping, chemical, or photoeffect) but no positive charges. Hmmm?!... - Step 1: Shoot the JELL–O atom! Have a heavy blob that seems like grape JELL–O, and you have gun with rubber bullets. How to find out what the middle of the blob is like? Have a heavy blob that seems like grape JELL-O, and you have gun with rubber bullets. How to find out what the middle of the blob is like? Shoot bunch of bullets into it and see this. What is the inside like? Have a heavy blob that seems like grape JELL-O, and you have gun with rubber bullets. How to find out what the middle of the blob is like? Shoot bunch of bullets into it and see this. What is the inside like? a. hollow b. solid JELL-O c. hard heavy core surrounded by JELL-O d. bunch of hard little objects distributed through blob Have a heavy blob that seems like grape JELL-O, and you have gun with rubber bullets. How to find out what the middle of the blob is like? Shoot bunch of bullets into it and see this. What is the inside like? Hard heavy core surrounded by JELLO Only one thing is reflecting bullets, sending them straight back so must be hard and heavy. Essentially Rutherford experiment and conclusion (TZD 3.12). Rutherford shot alpha particles = 2 protons, 2 neutrons n pn Positive charge Bullets = p The Rutherford experiment Detector rutherford-scattering_en.jar The Rutherford atom: Tiny nucleus with protons and neutrons (~99.98% of mass) Surrounded by large diffuse cloud of low mass electrons 10-10 m n n pnp p pn p 10-14 m The ‘Rutherford atom’ is a much better model than the ‘Thomson model’, but it is still far from helpful. A good model should: • explain how stuff works • predict the behavior of it • be quantitative • etc. à Need a better model! increasing energy What do the vertical arrows represent? allowed energies in an atom Reading Quiz “ground state” a. arrows on right represent absorption of light, arrows on left represent emission. b. arrows on both left and right represent emission c. arrows on both left and right represent absorption d. arrows on left represent absorption of light, arrows on right represent emission. increasing energy allowed energies ground state d. arrows on left represent absorption of light, arrows on right represent emission. Let’s see how atoms behave when shot-at! (with (with electrons) electrons) http://www.jstor.org/discover/ 10.2307/40677461? uid=3739568&uid=2129&uid=2 &uid=70&uid=4&uid=3739256& sid=21101136089553 What happens if we bash atoms with electrons? In atomic discharge lamps, lots of electrons are given kinetic energy (accelerated by a high voltage). When they bash into atoms some of this kinetic energy is transferred to the atom à Atom get's excited!! (“Neon” lights, Mercury street lamps) - 120V + Cathode (hot metal, so electrons can come out) Anode (positive potential) Note: ‘Anode’ and ‘Cathode’ have different meanings in physics or chemistry. Remember ‘Cathode Ray Tube’ (CRT): Electrons leave the cathode (in physics). Use a grating to look at the spectrum of the discharge lamps Hold grating only by edges...oil from hands ruins grating! Hold close to eye... See rainbow from lights. Turn grating so rainbow is horizontal. (Rainbow appears quite a bit to the side of the actual lamp. Hydrogen Na Hydrogen neon neon neon Hydrogen White light = whole spectrum. Each type of atom produces unique set of colors, called its “spectrum”. None of the atoms produces white light! Na Na Na NaNa Hydrogen Hydrogen Hydrogen Hg Hg Hg Hg Hg What colors from white light? What colors from neon? What colors from hydrogen? What from mercury? What from sodium? 400 500 600 Wavelength (nanometers) 700 800 Each type of atom produces unique set of colors. Discussion: Given that we now know that light comes quantized in photons with energy E=hc/λ= hf. What do these observations imply about electrons in atoms? Student suggestions: a. b. c. d. e. Each type of atom produces unique set of colors. Discussion: What does this imply about electrons in atoms? Implies that electrons only change between very specific energies. Each time a photon is emitted an electron must be changing in energy by that amount (releasing energy). Only way for individual atoms to give off energy is as light. Atoms are lazy - always want to go back to lowest energy state. 2. Excited atom ..electron 3. Electron 1. Fast electron in atom goes to higher jumps back to hits atom energy low energy Less KE e Higher energy e Ground state ~10ns Excited state e