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UT AST 301 - Lecture Notes

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AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteTopics for this weekWhat is a photon? What is an electromagnetic wave?How are the photon and wave pictures of light related?Make a sketch of an atom, showing its parts.How do the wave properties of electrons result in onlycertain electron orbits being allowed in an atom?How does the fact that only certain electron orbits canoccur result in photons of only certain wavelengthsbeing emitted?Describe emission and absorption line spectra and theconditions under which each occurs.Describe black body radiation and the relations betweentemperature and the power emitted and thewavelengths of light emitted.Describe the Doppler shift.Spectra of gasses and solidsWhen solids are heated they emit all wavelengths of light(a continuous spectrum).How bright the light at different wavelengths is depends onthe temperature of the solid. Hotter solids emit morelight of all wavelengths, but they especially emit moreshort wavelength (blue and violet) light.When gasses are heated they emit only certainwavelengths of light (an emission line spectrum).Different gasses emit different wavelengths.A cool object (gas or solid) can absorb some of the lightpassing through it.Hot solids – continuous spectraThe temperature of an object is a measure of how muchenergy its atoms have.Since atoms in hotter objects have more energy, they canemit photons with more energy than cooler objects can.(When an atom emits a photon the photon energy comesfrom the atom, so an atom can’t emit a photon with moreenergy than the atom had.)So hot objects emit high energy photons, or shortwavelength light.Since λ α 1/Ephoton and Ephoton ~ Eatom α T, λ α 1/TThey also emit more photons that cooler objects do.The rule is the amount of power emitted (energy emittedeach second) is Pemitted α T4QuizWhen you look at a light bulb through a piece of red plastic,the light looks red. This is because:A. The plastic absorbs the white light from the bulb andemits red light.B. The plastic shifts all of the photon wavelengths towardthe red end of the spectrum.C. The plastic absorbs the green photons and convertsthem into red photons.D. The plastic absorbs the green photons, leaving the red.Emission line spectraHot gas emits light of only certain wavelengths.If a source emitting a continuous spectrum lies behind coolgas, the gas absorbs some of the light, and it absorbsthe same wavelengths of light that it would emit if hot.To understand why gasses act this way, we need tounderstand more about how electrons orbit in atoms.What is an atom?A hydrogen atom has one proton at its center, with oneelectron orbiting around the proton.The proton has a positive electrical charge.The electron has a negative charge and is about 2000times less massive than the proton.Opposite charges attract, with a force law like that forgravity, so we expect the electron orbit to obey laws likeKepler’s laws.Other atoms have additional protons in their nuclei andadditional electrons orbiting around their nuclei.The also have neutrons (electrically neutral particles withmasses similar to proton masses) in their nuclei.Electron wavesWe normally think of electrons as particles.But like photons, they have both wave and particleproperties.The height of the wave describes the probability of findingthe electron in different places.The wavelength of the probability wave is related to theelectron speed, v, by:λ = h / mv,where h is Planck’s constant and m is the electron mass.In an atom, an electron must orbit at a distance from thenucleus so that an integral number of probability wavesfit around its orbit.This causes only certain electron speeds or energies to beallowed.Emission and absorption of light by atomsWhen an atom absorbs light, the photon energy must equalthe energy needed to make an electron jump from asmall orbit to a bigger one.When an atom emits light, a photon is created, and theenergy of the photon must equal the energy lost by theatom when an electron jumps from one orbit to another.A big jump for an electron requires a high energy photon,or short wavelength


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UT AST 301 - Lecture Notes

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