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.What is light?We need two ways of looking at light:It is an electromagnetic wave.We often think of radio waves this way.In fact they are just very long wavelength light.It is also a shower of particles called photons.x-rays are usually thought of as photons.They are very short wavelength light.For visible light we need both pictures.What is a wave?A moving disturbancelike a wave going around the stadiumor a wave in wateror a wave in a springFor all of these, no object moves with the wave.It is only the disturbance that is moving.Waves can be described by:wavelength – the distance between peaksperiod – the time between peaks passing a pointamplitude – the wave heightWhat is the relation between wavelength andperiod of a wave?Measure the wavelength and period of a wave in a spring,for several different frequencies.What is the relation between wavelength andperiod of a wave?In one period of a wave, one full cycle (peak through valleyand back to peak) passes by.That means that the wave has moved by one wavelength.Since speed is distance traveled / time spent, and a wavemoves a distance of one wavelength in a time of oneperiod, its speed must be:speed = distance / time = wavelength / period v = λ / pWe more often use frequency = 1 / period. Then:speed = wavelength x frequency v = λ x fWhat is light?We need two ways of looking at light:It is an electromagnetic wave.We often think of radio waves this way.In fact they are just very long wavelength light.It is also a shower of particles called photons.x-rays are usually thought of as photons.They are very short wavelength light.For visible light we need both pictures.What is a photon?Photons are small bunches of light waves.Each photon carries a certain amount of energy.What is energy?Energy is best defined by examples:a hot object has more energy than a cold one.a fast-moving object has more energy than a slow one.a rock at the top of a hill has more energy than one atthe bottom (because when it rolls down it will becomea fast-moving object).What is the energy of a photon?The concept of energy is useful because we have formulasfor each different type of energy.The energy of a moving object = ½ mass x speed2.The amount of heat energy in an object is proportional to itsabsolute temperature (Centigrade + 273).If a moving object slides to a stop, its energy of motion isconverted to heat energy.Einstein realized that the energy in a photon of light isproportional to the frequency of the wave in the photon,or inversely proportional to its wavelength:E = h f = h c / λ(h is a very small number, called Planck’s constant.)What is light?We need two ways of looking at light:It is an electromagnetic wave.It is also a shower of particles called photons.It is best to think of a photon as a wave packet – a bunch oflight waves.The mathematical connection between the two pictures oflight is given by:Ephoton = hc / λWhite light is a mixture of photons of different energies, orwaves of different wavelengths.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
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