2 Radiation and Spectra Chapter 5 LightRadiation and SpectraChapter 5
3 What is Light? Newton Maxwell Quantum Mechanics Prism shows white light contains all colorsLight made of particles (photons)MaxwellTheory of electricity and magnetismLight is electromagnetic wavesProduced by wiggling electronsRadiation = production of lightQuantum MechanicsLight is both: particle and wave
4 Waves Wavelength ( l ) Frequency ( f ) Distance between crests (or troughs)Frequency ( f )How often it repeats (wiggles up and down)Measured in Hertz (Hz)number of times per sec
5 Waves Speed c = 3 x 108 m/s Wavelength inversely related to frequency c = lfWavelength inversely related to frequencyl = c / fhigh frequency = short wavelengthlow frequency = long wavelength
6 Particles as Waves“Wave Packet”particle/photon = localized wave
7 Properties of Light Color Carries energy (heat) Depends on frequency blue = high frequency = short wavelengthred = low frequency = long wavelengthCarries energy (heat)Photon energyE = h fhigh frequency = high energy = bluelow frequency = low energy = redh = Planck’s constant
8 Red light has ____ than blue light. A. larger frequency, energy, and wavelengthB. smaller frequency, energy, and wavelengthC. larger frequency and energy, but smaller wavelengthD. smaller frequency and energy, but larger wavelength
9 All types of light travel at the same speed - Which of the following travels fastest?}radio wavesinfrared (heat) wavesmicrowavesblue light wavesnone of the aboveAll aretypes oflight!All types of light travel at the same speed -the “speed of light”, c
10 CPS Question The color of visible light is determined by its ____. A) brightnessB) amplitudeC) speedD) wavelength
11 CPS QuestionIf the wavelength of light increases, the frequency must ____.A) increase alsoB) decreaseC) remain unchanged
12 CPS QuestionThe bending of light that occurs when moving between media of different densities is called ___.A) reflectionB) refractionC) diffractionD) distortion
13 Propagation of Light Photons travel in straight lines energy spread over larger area at larger distancesproduces 1/r2 decrease in brightnessDouble distance - brightness decreases by 4
14 If a 100-watt light bulb is placed 10 feet away from you, and an identical 100-watt light bulb is placed 100 feet away from you, which will appear brighter?The closer oneThe farther oneThey will appear the same brightnessHow much fainter will the far one appear compared to the close one?Twice as faint10 times fainter100 times fainter1000 times fainter~ 1/r2
15 The Electromagnetic Spectrum 1 nm = m , 1 Angstrom = mc =f
16 Electromagnetic Spectrum Visible light:red, orange, yellow, green, blue, indigo, violet (ROYGBIV)Invisible Light:Ultraviolet = bluer than blueInfrared = redder than redOther wavelengths:Short: X-rays, gamma-raysLong: microwave, radio
17 Which kind of electromagnetic radiation has a wavelength longer than that of visible light?A. infraredB. ultravioletC. x-raysD. gamma raysE. none of the above
18 What’s the wavelength of my favorite radio station? Link to:
19 Thermal Radiation All objects radiate (thermal radiation) Objects made of atomsAtoms (and their electrons) vibrateWiggling electrons radiate, producing lightBigger objects produce more lightHigher temperature = stronger vibrationHotter objects emit more lightPerfect absorber is blackAbsorbed light (energy) heats objectTemperature increases untilemitted energy = absorbed energyEmitted radiation called Blackbody RadiationThermal radiation emitted by most objects similar to blackbody
20 What does the spectrum of an astronomical object's radiation look like? Many objects (e.g. stars) have roughly a "Black-body" spectrum:BrightnessFrequencyalso known as the Planck spectrum or Planck curve.
21 Blackbody Radiation Laws Luminosity, LL = energy emitted per secondLuminosity for a spherical object (a star)L = 4p R2 s T4R = radius (size) of star; T = temperaturedouble size, luminosity increases by 2x2 = 4double temperature; luminosity increases by 2x2x2x2 = 16Stefan-Boltzmann Law
22 Spectroscopy and Atoms How do you make a spectrum?
23 When you bend light, bending angle depends on wavelength, or color. Refraction of lightWhen you bend light, bending angle depends on wavelength, or color.
24 Questions How is temperature related to the amount of energy radiated? How is temperature related to the color of the object?(Blackbody Demo)
25 The wavelength of peak emission tells us the temperature of the object! "cold" dust"cool" starSun"hot" starsfrequency increases, wavelength decreases
27 Blackbody Radiation Laws ColorWavelength where most light emittedlmax = 3 x 106 / TT in Kelvin; lmax in nanometers (1 nm=10-9m)Cool stars are redHot stars are blueColor indicates temperature!Wien’s LawAs T , Wavelength , Color = redderAs T , Wavelength , Color = bluer
28 The graph above shows blackbody spectra for three different stars The graph above shows blackbody spectra for three different stars. Which of the stars is at the highest temperature?Star AStar BStar CBecause peak energy emissionoccurs at shortest wavelength
29 Doppler Shift Originally discovered using sound waves Moving object emits light with slightly different colorFrequency (pitch) of approaching object is higherBlueshiftWavelength shorter (shifted blueward)Frequency (pitch) of receeding object is lowerRedshiftWavelength longer (shifted redward)videoVideo
31 Spectroscopy Prism separates light into different colors Continuous spectrumcontains all colorsExample: blackbody spectrum
32 Spectroscopy Absorption Line Spectrum Some colors are missing (discrete lines)SolarSpectrumN.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
33 Spectroscopy Emission Line spectrum Only certain colors are present (discrete lines)Spectrum for each element unique (like fingerprints)
34 Pattern of lines is a fingerprint of the element
35 Sodium emission and absorption spectra For a given element, emission and absorption lines occur at the same wavelengths.Helium discovered in Sun’s spectrum before being found on Earth!Sodium emission and absorption spectra
36 Spectrum of the Sun Absorption spectrum What causes emission/absorption of light at specific wavelengths?InteractiveVideo 1, 2, 3
37 Types of Spectra1. "Continuous" spectrum2. "Emission" spectrum3. "Absorption” Spectrumvideo
38 The Particle Nature of Light Light interacts with matter as individual packets of energy, called photons.cphoton energy is proportional to frequency:E f (or E example: ultraviolet photons are more harmful than visible photons.1
39 Model Atom Nucleus contains protons and neutrons number of protons = element(1 proton = hydrogen, 2 protons = helium, etc.)number of neutrons about same as protonsIsotope = different number of neutronshydrogenheliumIsotopes of hydrogen
40 Model Atom Electrons orbit nucleus Number of electrons = number of protonsIonization = removing electronsOnly certain orbits are allowedhydrogenhelium
41 The Nature of Atoms (Fair Analogy) The Bohr model of the Hydrogen atom:electron__++proton"ground state"an "excited state"(Fair Analogy)
42 Atomic Absorption Atom absorbs photon energy electron “jumps” to higher energy orbitonly certain discrete orbits are allowedAtom can absorb only discrete colors (energies)
43 When an atom absorbs a photon, it moves to a higher energy state briefly When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system!
44 Atomic Emission Electron “jumps” to a lower energy orbit Atom emits photoncan emit only discrete colorssame colors (wavelengths/energies) as absorption
45 Atomic Energy Levels Energy Levels Different for each element each element has unique set of absorption/emission lines
46 Other elements Helium Carbon neutron protonEach element has its own allowed energy levels yielding a unique spectral fingerprint.
47 Kirchoff’s Laws Continuous spectrum Emission line spectrum Produced by hot solid (or dense gas)Emission line spectrumProduced by hot, low density gasAbsorption line spectrumProduced when continuous source is viewed through cooler low density gas
48 Kirchoff’s Laws Absorption lines same wavelengths as emission lines Gas can only absorb and emit at certain discrete frequencies/wavelengths/energiesvideo
49 If you analyze the light from a low density object (such as a cloud of interstellar gas), which type of spectrum do you see?dark line absorption spectrumbright line emission spectrumcontinuous spectrum
50 Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight?dark line absorption spectrumbright line emission spectrumcontinuous spectrum
51 CPS QuestionWhich ONE of these is constant for all forms of EM radiation in a vacuum?A) amplitudeB) wavelengthC) frequencyD) speedE) energy
52 CPS Question Which ONE is NOT a property of a blackbody? A) It appears black, regardless of its temperature.B) It emits radiation in a continuum of wavelengths.C) Its spectrum peaks at a wavelength determined by its temperature.D) The total energy that it radiates increases rapidly with temperature.
53 CPS Question The Sun's observed spectrum is _____. A) A continuum with no lines, like the rainbow.B) A continuum with bright emission lines.C) Only absorption lines on a black background.D) Nearly a continuum with some absorption lines.
54 Ionization Hydrogen _ _ + + _ _ Helium + + + + _ _ "Ion" Absorbing a high energy photon and atomic collisions can both lead to ionization.
55 Spectrum of the Sun Complicated objects => many different elements Nearly continuous absorption spectrumWhat causes emission/absorption of light at certain wavelengths?
56 . . . . . . Why emission lines? hot cloud of gas - Photon absorption/atomic collisions excite atoms- Electron drops back to lower level- Photons at specific frequencies emitted
60 QuestionHow does the pitch or tone of a sound wave change when the source of the sound is moving towards or away from you?What about when you are moving towards or away from the source?Does this effect occur for all types of waves or just for sound waves?