Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Q1: What do the vertical arrows represent? allowed energies in an atom increasing energy Exam 2 is next week (Tuesday 10/18, 7:30-9pm) Do you want this exam to be cumulative or only cover the new material? (The final will be cumulative) A) Cumulative (1/3: SR, 2/3: chapters 3-5+lasers) : 18 votes : 34 votes B) Only new material (chapters 3-5+lasers) “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. e. I don’t know. I have never seen such a diagram. What we know so far about atoms: p n nn p p p nn nn p Rutherford: Atoms have a tiny, but very dense core surrounded by a cloud of electrons. a.The electrons in the discharge lamp hit the neon atoms with higher kinetic energy than the electrons hit the sodium atoms 120V Discharge lamps: Atoms struck by fast electrons emit light of distinct colors. b.The electrons in the discharge lamp hit the neon atoms with less speed than the electrons hit the sodium atoms Energy levels: Electrons in atoms are found only in discrete energy levels. When they jump down to a lower level, a photon is emitted carrying away the energy. ∆E ∆E=hf e Different Atoms: Different types of atoms have different level structures (seen by the distinct set of colors they emit): Neon: Strong red line; Sodium: strong yellow line > e e Energy Energy level diagrams represent energy levels the electron can go to. Different height = different energy e e Q2: A neon lamp emits a red line. Sodium emits a yellow line. What accounts for this difference? e c.The energy spacing between the electronic energy levels that are responsible for these lines are smaller in the neon atom than in the sodium atom. d.The energy spacing between the electronic energy levels that are responsible for these lines is larger in the neon atom than in the sodium atom. Q3: An single electron bashes into an atom in a discharge lamp Electron leaves hot filament with nearly zero initial kinetic energy + 10 V e -2 eV -3 eV -6 eV For Hydrogen, transitions to ground state in deep ultraviolet! No light emitted with colors in this region because no energy levels spaced with this energy. -10eV If atom fixed at the center of the tube, list all the possible photon energies (colors) that you might see? A. 1eV, 2eV, 3eV, 4eV, 7eV, 8eV B. 4eV, 7eV, 8eV C. 1eV, 3eV, 4eV D. 4eV E. Impossible to tell. Investigate energy levels in atoms Some handy relations: Current flowing through a wire: 1 Ampere = 1 Coulomb / Second 1 Watt = 1 Ampere 1 Volt = 1 C/s 1 V 1 Joule = 1 Watt 1 Second = 1 C 1 V 1C 0V i.e: An electric potential of 1 V puts in one Joule of energy into a charge of 1 C kinetic energy increases by 1 Joule. 1V + + + + An e- has a charge of 1.610-19C, therefore, it gains a kinetic energy of 1eV=1.610-19J Q4: Q5: energy levels of electron in atom 3 2 1 energy of colliding electron G (ground) If the colliding electrons have an energy between that of level 2 and level 3 when they hit the atom how many different colors could be emitted by the atom? a. no levels will be excited, and so no light will come out. b. 1 color of light will come out c. 2 colors of light will come out d. 3 colors of light will come out e. 4 colors come out. Applications of atomic spectroscopy 1. Detecting what kind of atoms are in a material. (excite by putting in discharge lamp or heating in flame to see spectral lines) 2. Detecting what the sun and the stars are made of: Look at the light from a star through a diffraction grating. See what lines there are; Match up to atoms on earth. When an emission line appears very bright that means: a.Multiple photons are emitted for each single electron transition b.More electron transitions are occurring each second c.The electronic energy levels are farther apart and thus the line appears brighter d.a and b e.b and c Atomic spectra in astronomy L Let’s investigate the internal working of a galaxy L telescope star diffraction grating 3. Making much more efficient lights! Incandescent light bulbs waste >90% of the electrical energy that goes into them! (<10% efficient) Streetlight discharge lamps (Na or Hg) ~80% efficient. Fluorescent lights ~ 40-60% efficent. R R longer wavelength lower frequency Spectral lines from atoms in stars Hubble and the big bang Spectral lines from Hydrogen Application: Designing a better light What is important? (Well, we have to be able to see the light!) What color(s) do you want? (red, green & blue is what our eyes can see) How do you excite the atoms to desired level? ( Electron collisions) How to get electrons with desired energy when hit atoms? What determines energy of electrons? Edwin Hubble, PNAS March 15, 1929 vol. 15 no. 3 168-173 Incandescent light (hot filament) Temperature = 2500-3000K Hot electrons jump between many very closely spaced levels (solid metal). Produce all colors. Mostly infrared at temp of normal filament. >90% is worthless Infrared radiation (IR = longer than ~700nm) Only certain wavelengths emitted. HV Right atom, right pressure and voltage, mostly visible light. IR P Discharge lamp Energy levels in isolated atom: Streetlight discharge lamps (Na or Hg) 80% efficient. λ ~10% of energy is useful visible light Florescent Lights. Discharge lamp, White= Red + green +blue 40-60% efficient (electrical power⇒visible light) Converting 180nm UV light into visible photons with “phosphor”. phosphor coating Hg 180 nm far UV Hg Hg Hg Hg e Hg discharge lamp/flor. tube 120 V real phosphors more than just 3 phosphor wastes 20-30% energy ⇒ heat energy of electron in phosphor molecule Florescent Lights. Discharge lamp, but want to have it look white. White = red + green +blue 40-60% efficient (electrical power ⇒ visible light) How to do this? Converting UV light into visible photons with “phosphor”. Phosphor converts 180 nm UV to red+ green+blue. 180nm 6.9 eV energy per photon 633nm (red) 2 eV / photon 532nm (green) 2.3 eV / photon 475nm (blue) 2.6 eV / photon } 6.9 eV phosphor wastes 20-30% energy ⇒ heat Answers to clicker questions Q1: D Q2: C Q3: D (Note that the atom is in the middle of the tube!) Q4: D Q5: B (Each electron transition produces exactly one photon with energy equal to energy difference between electron energy levels. More electron transitions, more photons, brighter line. )