Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day• How would you describe the difference between temperature and energy?• What are ways that energy can transform from one form to another?• What is light?• How do energy and wavelength vary along the electromagnetic spectrum?• What are the properties of a thermal spectrum?• How does the color of an object change with temperature?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW2Energy is not the same as Temperature! •Temperature: indicates typical speed of particles.•Energy: related to both speed & mass.Same Temperature, Different Total EnergyAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Same temperature, Different energy Water molecules are moving with the same speeds in both, but there’s less energy in the cup. V is the same, but M is different.[Energy per particle is the same, but more particles in the urn]Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4Deep fact:Energy is always conservedIt can change from one form to another, or be transported from place to place, but it never disappears.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5Changes in temperature are usually associated with the transfer of energyHeat (i.e. energy) transfers to water through conduction.Increased energy increases the water’s temperature, and changes its stateAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6How does energy move from place to place?1. CONDUCTION:Energetic particle whaps another energetic particle, which whaps another energetic particle, which whaps another one…Matter stays in about the same place, Energy moves!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7Examples of conduction:Cold here initially, but gradually warms up.Metal in pot conducts heat from stove to water.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8How does energy move from place to place?2. CONVECTION:Energy physically moves from one place to another! Both Energy and Mass transport spatiallyENERGYAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9Examples of convection:Lava heats up in bottom of the lamp, and rises.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10How does energy move from place to place?3. RADIATION:Energy is transported by light.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11Radiation:“Bad” radiation is light which has so much energy that it can damage what it runs into. •Emission of Light = “Radiation”We use a more general definition of radiation ! any light emitted from an object. Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12The Facts of Light•Light sometimes acts like a massless particle“PHOTON”Goes in a straight line unless it interacts with something else (i.e. is reflected, absorbed, or scattered).Paths of individual photonsYou only “see” the photons that happen to enter your eyeAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13The Facts of Light•Photons are “discrete”. You can count when individual photons arrive.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14Photons are discrete.X-ray image of the moon, showing individual photons detected.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15The Facts of Light•Light also acts like a wave.Complex patterns can be built up from many waves (“superposition”)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16In this class we’ll often mush both views together.• We’ll talk about emission and absorption of individual photons.• But, we’ll often assign wave-like qualities to the photon:1.Wavelength2.Frequency3.Energy4.VelocityAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW171. Wavelength•The distance over which the wave pattern repeats is the wavelength.! is the symbol for wavelength“lambda”Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW181. Wavelength•The color of light depends upon its wavelength.Measured in “nanometers” (10-9 m) or “angstroms” (10-10 m) • Short wavelengths areBlue• Long wavelengths areRedAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW191. Wavelength•Complex colors can be built from a superposition of wavelengths (i.e. light of different colors).PS. A prism bends light of different wavelengths by different angles, spreading the “spectrum”.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW211. Wavelength•We see with our eyes only a tiny fraction of the range of possible wavelengths“Logarithmic scale”: each tic mark represents a factor of ten increase. Size of atomic nucleus!Size of Mt. Everest!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW221. WavelengthAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW231. Wavelength•A “spectrum” indicates how much light is emitted at each wavelengthPicture of what you’d see if the light were passed through a prism or “diffraction grating”Graph of brightness versus wavelength (i.e. color) of the spectrumAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW242. Frequency•The rate at which peaks pass a given location is the frequency." (greek “nu”) is the symbol for frequencyMeasured in “Hertz”, which has units of 1/second. One crest in one second is 1 Hertz, Two crests in one second is 2 Hertz, etc.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW253. Energy•The energy of a photon depends entirely on its frequency (of wavelength)(Energy is “proportional” to frequency, and “inversely proportional” to wavelength)FREQUENCYWAVELENGTHEnergy isgreater when…..Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW263. EnergyDangerous!!!Burns you! Goes through you!Mostly Harmless.(Microwaves only hurt because they have a peculiar interaction with water)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW273. Energy•Watch your intuition here!BLUE• Short wavelength• High frequency • HIGH ENERGYRED• Long wavelength• Low frequency • LOW ENERGYAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW284. Velocity•The speed of light is constant.•The speed of light is closely related to its wavelength and its frequency.ORYou always know the
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