Wednesday Feb. 9Syllabus and class notes are at: www.as.utexas.edugo to courses, AST301 – Introduction to Astronomy – LacyFor this week you should have read Chapters 5.1 and 6If you want help on homework or anything covered in thecourse, come to discussion session Thursday at 6 PM inRLM 15.216B.Or TA or prof. office hours (see syllabus)Topics 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.Newton’s version of Kepler’s 3rd lawNewton’s laws can be used to derive Kepler’s 3rd law.A force is needed to cause a planet to move on a curvedpath. That force is the force of gravity of the Sun.By equating the force of the Sun’s gravity to the forceneeded to cause a planet to follow a curved path, we cancalculate the speed of the planet.The result is: v = (G MSun / a)1/2,where v is the average speed of the planet, MSun is themass of the Sun, and a is the average distance of theplanet from the Sun.Knowing the speed of the planet, we can calculate the timeit takes to orbit the Sun.The result is: P2 α a3 / MSunWhat 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, nothing physical 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 = _ 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.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
View Full Document
Unlocking...