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Reminders • Office hours: 1-2pm, July 18 (today!) Watanabe Room 403 • • Review Session tomorrow Exam on Friday, 50 multiple choice questions, 75 minutes Today: Emission/Absorption Lines and the causes Redshift and Blueshift of light Learning from light: Spectra • Spectrum: different amounts of light from an object at different wavelengths • Thermal Blackbody - one type of continuous spectrum • The shape of spectrum depends on temperature only Learning from light: Spectra Continuous Emission line Absorption line Learning from light: Spectra Spectrum: different amounts of light from an object at different wavelengths A white light bulb will show emission at many wavelengths: continuous spectrum A sodium bulb (filled with sodium gas) will emit light only at very specific wavelengths: emission line spectrum If we put a cloud of sodium gas between us and the white bulb, we will see most colors but some specific wavelengths will be missing: absorption line spectrum Emission line spectrum Argon • • We see very specific colors of light emitted from a low density cloud of gas. We call these features emission lines. Lines we see depend on compositions, density and temperature Hydrogen Helium Neon Absorption line spectrum • Dark parts of continuous spectrum (“missing” light) are absorption lines. • Depend on composition, density • For a given element, absorption and emission lines are at same wavelengths What cause Em/Abs lines? Basic Atomic Physics • Basic chemical elements (hydrogen, oxygen, carbon...) are made up of unique combinations of: Protons Neutrons Electrons Basic Atomic Physics • • Atoms are TINY: 10-10 meters! How many atoms do you think there are in one drop of water? 1021 = 1,000,000,000,000,000,000,000 atoms 2/3 hydrogen, 1/3 oxygen Basic Atomic Physics • Electrons can only appear at certain orbits (carrying certain energies) in the atom --- Energy levels in atoms • The configuration of orbits depend on elements (e.g. Hydrogen and Helium are different) Basic Atomic Physics • Energy levels in atoms are quantized • Strange but true: electrons in atoms can only have specific amount of energy • • Like rungs on a ladder Electrons can go up and down only between these levels Basic Atomic Physics • Lowest energy level of an electron is called its ground state • Energies of electrons in unit called electron-Volts, or eV • Ground state has energy of 0 eV • Higher levels are called excited states • Each excited state requires a specific energy to get to Basic Atomic Physics • How do these electrons go up and down at these orbits? • Recall: light has property of particle, and we call them as photons • A photon carries a particular amount of energy that depends on its frequency • The higher the frequency of the photon, the more energy it carries The electron can go up and down at the orbit with emit or absorb photons (carrying specific amount of energy) Energy states in Hydrogen Class Action! Which atom would be absorbing light with the greatest energy? Class Action! Which atom would emit light with the longest wavelength? Questions? What causes em/abs lines? • As atoms bump into each other, they can transfer energy • If bump is of just the right energy, it can cause an electron to jump between energy states • Electron jumps back to lower state by getting rid of its newly acquired energy • Emit photon (light) with the exact amount of energy to “relax” from excited state • Energy must be conserved! Why are these lines useful? • Every atom has its own specific set of possible transitions • So each emits and absorbs at different specific wavelengths • Creating unique spectra we can use as fingerprints to identify what something is made of! Argon Hydrogen Helium Neon What causes emission lines? • As long as gas is warm enough, the atoms bump into each other often • • Continuously exciting electrons • • Creates emission line spectrum That give off photons as they relax As gas gets hotter, bumps have more energy Emission line spectrum Continuous Emission line Absorption line What causes absorption lines? • Illuminate a cold gas (most atoms in ground state) • Atoms can absorb photons of just the right energy to excite electrons • The light at these energies (wavelengths) does not make it to the observer • When electrons relax, photons emitted in all directions, not just along line of sight, so the absorption lines don’t all fill in Absorption line spectrum Continuous Emission line Absorption line Class Action! Class Action! Questions? Learning more from light • We’ve seen how to find out what makes up a star or galaxy by using emission/absorption lines • • How hot something is based on its color • What else can we learn? Based on the Blackbody radiation, we can learn the sizes and temperatures of stars Learning more from light • • • How fast something is moving towards or away from us Using a “light” version of the Doppler effect We are familiar with this effect for sound... The Doppler effect Doppler effect for light • If something is moving towards us, the emitted light wave peaks are closer to each other than if it was standing still • The wavelength of light we observe will be shifted to shorter wavelengths, which we call a blueshift Doppler effect for light • If something is moving away from us, the emitted light wave peaks are further apart than if it was standing still • The wavelength of light we observe will be shifted to longer wavelengths, which we call a redshift Measuring redshift • What kind of spectrum has features at specific wavelengths that we can use to measure the red/ blueshift of an object • Emission or Absorption What redshift doesn’t tell us.. • Doppler shift only tells us about component of object’s motion toward or away from us • We call this the radial direction • It tells us nothing about the motion in other directions Class Action! Class Action! Questions? Lecture Tutorial Break up into groups of 2-3 NO MORE THAN THREE In your group, work through the following: Doppler Shift (pages 73 - 77) Discuss the answers – don’t be silent! Nick and I will be roaming around if you need help…