1Announcements• Homework 5 available tomorrow at about 2 PM;due on Sunday, Nov. 16 before midnight• No class on Tuesday, Nov. 11 (Veteran’s Day)• Midterm 2 on Wednesday, Nov. 12• We’ll cover telescopes after the exam• Study materials available late Weds. or earlyThursday, including practice exam on OWLImages of Enceladus obtainedlast Friday (10/31/2008):Radar image of Saturn’smoon Titan obtainedyesterday (11/03/08)• This appears to be ariver bed of some sort.• At one end, the riverbed is about 3 milesacross.• How do we know this?2For an object with a fixed size (a fixedlinear diameter), its angular diameterdecreases when the object’s distance isincreased.Angle (in radians) = linear diameterFull circle = 360°Full circle = 2π radians(π = 3.141592654)360° = 2π radiansdistance• This appears to be a riverbed of some sort.• At one end, the river bedis about 3 miles across.• How do we know this?• Measure the angle fromone side of the river bedto the other• Get distance to groundusing radar• Solve for linear diameterBut what is radar??Radar is one of the manyexamples of harnessingthe information containedin light.Hurricane observed with radar in 1960Light is:• Energy– Sometimes called “radiant energy”– Think: how does solar power work?• Information– The signal received by your car radio– The signals received by telescopes staring atstars– The signals received by your eyes right now!3Light is:• Energy– Sometimes called “radiant energy”– Think: how does solar power work?• Information– The signal received by your car radio– The signals received by telescopes staring atstars– The signals received by your eyes right now!Question: how does your cell phone work? What happens when yousend out a call? Answer: it sends and receives light.Basic Properties of Light• Life: is it bitter, or is it sweet?• Who knows -- you’ll have to findout for yourself.• But what about light? Is it aparticle or is it a wave?Interaction of Light with Matter• Remember that eachelectron is only allowed tohave certain energies in anatom.• Electrons can absorb lightand gain energy or emitlight when they loseenergy.• It is easiest to think of light as a particle whendiscussing its interaction with matter. The emitted packetof light is called a photon.• Only photons whose energies (colors) match the “jump”in electron energy levels can be emitted or absorbed.HydrogenEach element has a uniquespectrum. The spectrum isa fingerprint.Therefore, by studying the spectrum of an object, we can determineits composition.4Atoms can be excited by light...If a photon ofexactly the rightenergy is absorbedby an electron in anatom, the electronwill gain the energyof the photon andjump to an outer,higher energy orbit.A photon of the same energy is emitted when theelectron falls back down to its original orbit.Or atoms can be excited bycollisions.Collisions (like in ahot gas) can alsoprovide electronswith enough energyto change energylevels.A photon of the same energy is emitted when theelectron falls back down to its original orbit.Three types of spectra1 A hot, dense glowing object (solid or gas) emits acontinuous spectrum. Continuum emission2 A hot, low density gas emits light of only certainwavelengths. Emission lines3 When light having a continuous spectrum passesthrough a gas, dark lines appear in the continuousspectrum. Absorption lines3 types of spectra5Some Energy Levels of a HydrogenAtomDifferent allowed “orbits” or energylevels in a hydrogenatom.Emission line spectrumAbsorption line spectrumSpectral Lines of Some Elements Neon Sodium Mercury Helium ArgonSpectral lines are like a cosmic barcode system for elements.Consider a rocktossed into a pond...Waves in waterWhat happens when a wave moves through a pond from the left side tothe right side?1. The water molecules move from the left sideto the right side.2. The wave moves from left to right, but thewater particles move from right to left3. The wave moves from left to right and thewater molecules move up and down butdon’t move sideways.6Properties of Waves• Wavelength – the distance between crests (ortroughs) of a wave.• Frequency – the number of crests (or troughs)that pass by each second.• Speed – the rate at which a crest (or trough)moves.For all Waves: Frequency =SpeedWavelengthLight: a “transverse” waveLike this piece of rope,the vibrations in a lightwave are transverse (i.e., perpendicular)to the wave’s direction. Unlike the wave at left, light does not require a “medium” such as airto propagate through. Light can travel throughempty space.(begins here)Sound: a “longitudinal” waveLike the spring at left,the vibrations in a soundwave are along the directionof motion of the wave. A sound wave does require a medium such as airto propagate through. Hence, there is no soundin outer space.(begins here)Resonance• When a cyclic(repetitive) is driven atjust the right frequency,it responds moredramatically. Thatspecial frequency is aresonance.• Resonances areparticularly importantwith wave phenomenaThe “orbital resonance” of some ofthe moons of Jupiter7Uniquely, light waves cantravel through emptyouter space.WHY?Most waves require sometype of medium to travelthrough, e.g., water wavesmust travel through water.Light waves• How does light propagate throughempty space?• Light is like electrons on a rope.• Electrons have a negative charge andtherefore exert an electromagneticforce; the closer the electron is, thestronger the force. We call the forcefield from an electric charge theelectric field.• A change of the electric field creates amagnetic field.• A change of the magnetic field, inturn, creates a new electric field.• Light carries itself along by itsown bootstraps!• Light is also know as anelectromagnetic wave. It is a seriesof changing electric and magneticfields that propagate through space.e- e-Charges closetogether causesstrong repulsiveforceCharges fartherapart leads toweaker repulsiveforceLight waves• For a wave, its speed:s = f λ• But the speed of lightis a constant, c.• For light: f λ = c• The higher f is, thesmaller λ is, and viceversa.• Our eyes recognize f(or λ) as color!Wavelength = λLight as a Particle• Light can also be treated as particles called photons –packets of radiative energy.• Photons are massless, however they have momentum andthey react to a gravitational field!• The energy carried