Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Clicker Question:Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Electromagnetic Radiation(How we get most of our information about the cosmos)Examples of electromagnetic radiation:LightInfraredUltravioletMicrowavesAM radioFM radioTV signalsCell phone signalsX-raysRadiation travels as waves.Waves carry information and energy.Properties of a wavewavelength ()crestamplitude (A)velocity (v)trough is a distance, so its units are m, cm, or mm, etc.Period (T): time between crest (or trough) passagesFrequency (): rate of passage of crests (or troughs),  Also, v = h1T(units: Hertz or cycles/sec)Demo: making waves - wave tableDemo: slinky wavesWavesRadiation travels as Electromagnetic waves.That is, waves of electric and magnetic fields traveling together.Examples of objects with magnetic fields:a magnetthe EarthClusters of galaxiesExamples of objects with electric fields:Protons (+)Electrons (-)}"charged" particles that make up atoms.Power lines, electric motors, …Scottish physicist James Clerk Maxwell showed in 1865 that waves of electric and magnetic fields travel together => traveling “electromagnetic” waves.The speed of all electromagnetic waves is the speed of light. c = 3 x 10 8 m / sor c = 3 x 10 10 cm / sor c = 3 x 10 5 km / sSunEarthlight takes 8 minutes c =or, bigger  means smaller c =1 nm = 10 -9 m , 1 Angstrom = 10 -10 mThe Electromagnetic SpectrumDemo: white light and a prismA SpectrumAll waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.Refraction of lightClicker Question:Compared to ultraviolet radiation, infrared radiation has greater:A: energyB: amplitudeC: frequencyD: wavelengthClicker Question:The energy of a photon is proportional to its:A: periodB: amplitudeC: frequencyD: wavelengthClicker Question:A star much colder than the sun would appear:A: redB: yellowC: blueD: smallerE: largerWe form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light).BrightnessFrequencyalso known as the Planck spectrum or Planck curve.What does the spectrum of an astronomical object's radiation look like?Many objects (e.g. stars) have roughly a "Black-body" spectrum:• Asymmetric shape• Broad range of wavelengthsor frequencies• Has a peakcold dust hotter star (Sun)“cool" starfrequency increases, wavelength decreasesApproximate black-body spectra of astronomical objects demonstrate Wien's Law and Stefan's Lawvery hot starsLaws Associated with the Black-body SpectrumStefan's Law: Energy radiated per cm2 of area on surface every second  T 4 (T = temperature at surface)Wien's Law:max energy 1T(wavelength at which most energy is radiated is longer for cooler objects) 1 cm2BetelgeuseRigelBetelgeuseThe total energy radiated from entire surface every second is called the luminosity. Thus Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)For a sphere, area of surface is 4R2, where R is the sphere's radius.The "Inverse-Square" Law Applies to Radiationapparent brightness 1D2D is the distance between source and observer.Each square gets 1/9 of the lightEach square gets 1/4 of the lightThe Doppler EffectApplies to all kinds of waves, not just radiation.at restvelocity v1 velocity v1 velocity v3 fewer wavecrests per second => lower frequency!velocity v1 velocity v2 you encounter more wavecrests per second => higher frequency!Demo: buzzer on a moving armDemo: The Doppler BallDoppler EffectThe frequency or wavelength of a wave depends on the relative motion of the source and the observer.1. Refraction Waves bend when they pass through material of different densities.swimming poolairwaterprismairairglassThings that waves do2. Diffraction Waves bend when they go through a narrow gap or around a corner.3. Interference Waves can interfere with each otherQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Rainbowsrred orange yellow green blue violetWhat's happening in the cloud?Sun's rayraindrop42o 40oDouble RainbowsClicker Question:Compared to blue light, red light travels:A: faster in a vacuumB: slower in a vacuumC: at the same speed in a vacuumClicker Question:Which of the following is not an electromagnetic wave:A: radio wavesB: visible lightC: X-raysD: sound wavesE: gamma-raysClicker Question:If a star is moving rapidly towards Earth then its spectrum will be:A: the same as if it were at restB: shifted to the blueC: shifted to the redD: much brighter than if it were at restE: much fainter than if it were at restComputer simulations of InterferenceRadiation travels as waves.Waves carry information and energy.Properties of a wavewavelength ()crestamplitude (A)velocity (v)trough is a distance, so its units are m, cm, or mm, etc.Period (T): time between crest (or trough) passagesFrequency (): rate of passage of crests (or troughs),  Also, v = h1T(units: Hertz or