Lecture 9: Walking the Planck: Thermal and Line Radiation

"Particle man, Particle man, doing the things a particle can. What he's like, its not important. Nobody knows, Particle man."

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    • Date: September 29, 1994
    Reading Assignment: pp. 74-90

    Description : Photoelectric Effect,Thermal Radiation, the Planck curve, the Doppler Effect, and line spectra.

    Objectives

  • Understand the photoelectric effect and why it is important. / Understand the concepts of intensity, flux and temperature. / Understand thermal radiation, including Wien's Law and Planck's law / Understand the Doppler effect and how it changes wavelength / Understand that line spectra are different than continuum spectra

    • Lecture Outline

    Slide # 1: Lecture 9 : Walking the Planck

  • photoelectric effect
  • thermal radiation
  • Doppler shift
  • spectral lines
    • Slide # 2: Lecture 8: Waves and Particles, A Light Motif
  • Waves are oscillations in space and time
  • Light has wavelike properties
  • Frequency and wavelength are mathematically related
  • Light is just one part of the EM spectrum
  • The atmosphere absorbs most of the other parts of the EM Spectrum
    • Slide # 3: Light
  • Almost everything we know comes from the analysis of starlight
  • No information can travel faster than light
    • Slide # 4: The Nature of Light
  • Light is neither a wave or a particle
  • Wave-like properties
  • Particle-like properties
    • Slide # 5: Light
  • A special kind of radiation
  • Only one part of the Electromagnetic Spectrum
    • Slide # 6: Why do we say light has Òwave-likeÓ properties
  • Diffraction
  • Interference
    • Slide # 7: Diffraction
  • ability of light to bend around corners
    • Slide # 8: Interference
  • ability of waves to reinforce or cancel each other
    • Slide # 9: The Electromagnetic Spectrum
  • Radio
  • Infrared
  • Visible
  • Ultraviolet
  • X-ray
  • Gamma-ray
    • Slide # 10: Atmospheric Opacity
  • some wavelengths are blocked by the atmosphere
    • Slide # 11: Particle Nature of Light
  • Light sometimes behaves like a particle
    • Slide # 12: The Photoelectric Effect
  • a test of the particle-like nature of light
    • Slide # 13: Photoelectric Effect
  • The Intensity of light does NOT affect the energy of particles
    • Slide # 14: Photoelectric Effect
  • changing the COLOR of the light bulb changes the ENERGY of the particles
    • Slide # 15: Photoelectric Effect
  • particle energy is proportional to frequency
    • Slide # 16: The Photoelectric Effect
  • Energy of photons is proportional to frequency
  • Energy of photons does not depend on amplitude
    • Slide # 17: Units Review
  • Hertz (Hz) = cycles per second
  • Angstroms = 10-10 meters = 10-8 cm
  • Ergs - unit of energy
  • very small unit
  • 100 watt light bulb = 109 erg/second
    • Slide # 18: The Photoelectric Effect
  • Blue Light
  • wavelength = 4000 Angstroms
  • frequency = 7.5 x 1011 Hz
  • Energy/photon =5.0 x 10-15 ergs
  • Red Light
  • wavelength = 7000 Angstroms
  • frequency = 4.3 x 1011 Hz
  • Energy/photon = 2.8 x 10-15 ergs
    • Slide # 19: Planck's Constant - h
  • 6.63 x 10-27 erg seconds
  • relates frequency in Hz to energy in ergs
    • Slide # 20: Photoelectric Effect
  • Discovered by Albert Einstein
  • Einstein receive Nobel Prize for this work (1921)
    • Slide # 21: Light has both Particle and Wave Natures
  • Light is actually not a particle or a wave
    • Slide # 22: A Few New Terms
  • Intensity
  • Temperature
    • Slide # 23: Intensity
  • the strength of radiation
  • how bright something appears
  • proportional to number of photons per second per unit area
    • Slide # 24: Temperature
  • a measure of an object'ss heat
  • Kelvin scale = Celsius Scale + 273
  • Nothing is below zero Kelvin
    • Slide # 25: Thermal Radiation
  • Intensity is only related to temperature
  • Independent of the type of material
    • Slide # 26: Spectra
  • A plot of Intensity vs Frequency
    • Slide # 27: Thermal Radiation Spectra
  • The Planck Curve is one type of spectra
  • curve of a perfect thermal emitter
  • Also known as the "black-body curve"
    • Slide # 28: A Planck Curve
  • the signature of thermal emission
    • Slide # 29: The Planck Curve
  • The Shape of the Curve is Independent of
  • Temperature
  • Composition
    • Slide # 30: The Planck Curve
  • The position of the peak and the area under the curve depend on Temperature
    • Slide # 31: The Planck Curve
  • Two laws describe the Planck Curve
  • Wien's Law
  • Stephan's Law
    • Slide # 32: Wien's Law
  • Describes the wavelength of the Peak of the Planck Curve
    • Slide # 33: Wien's Law
  • Peak Wavelength in cm
  • T in Kelvin
    • Slide # 34: Wien's Law
  • changing temperature changes the peak wavelength
    • Slide # 35: Wien's Law
  • If you can measure the Peak Wavelength, you can determine the Temperature
  • The peak wavelength is determined using spectroscopy
    • Slide # 36: Stephan's Law
  • Describes the flux from a black-body
  • Flux is the
  • amount of energy
  • per second
  • per square centimeter
  • A measure of how bright a Surface appears
    • Slide # 37: Stephan's Law
  • energy radiated depends on the fourth power of Temperature
    • Slide # 38: Stephan's Law
  • sigma is
  • Stephan-Boltzmann constant
  • 5.67 x 10-5 erg/s/cm2/K4
    • Slide # 39: Stephan's Law Slide # 40: Applications of the Planck Curve
  • The Sun is approximately a black-body
  • temperature is about 6000K
  • Planets emit most of their infrared energy as black-bodies
  • peak of the spectrum tells us the planet's average temperature
    • Slide # 41: Planck Curves
  • one way to tell the temperature and flux of distant objects
    • Slide # 42: The Doppler Effect
  • Motion can affect the wavelength and frequency of light
  • Motion does NOT change the speed of light
    • Slide # 43: Motion and Wavelength
  • Imagine traveling toward a light emitting source
    • Slide # 44: Motion and Wavelength
  • Observer #2 encounters more peaks per second than observer #1 due to his motio
    • Slide # 45: Motion toward a source
  • The moving observer encounters more peaks per second
  • The frequency of the light is HIGHER for the observer moving toward the light
    • Slide # 46: Motion Away from a Source
  • The number of peaks per second is less for an observer moving away from a ligh
  • The frequency is LOWER for an observer receding from the source
    • Slide # 47: Doppler
  • Blue Shift
  • higher frequencies, longer wavelength
  • the observer moves away from the source
  • the source moves away from the observer
  • Red Shift
  • lower frequencies, shorter wavelengths
  • the observer moves toward the source
  • the source moves toward the observer
    • Slide # 48: Doppler Effect
  • the change in wavelength is proportional to velocity
    • Slide # 49: Doppler Effect
  • the ratio of apparent and true wavelength depends of velocity
    • Slide # 50: Line Emission
  • Not all electromagnetic emission is thermal radiation
  • thermal radiation is one type of a continuous spectra
    • Slide # 51: Continuous and Line Spectra
  • Continuous Spectra have no spectral lines
    • Slide # 52: What produces spectral lines?
  • Atoms!
  • the location and strength of spectral lines depend on
  • type of atom or molecule
  • temperature
    • Slide # 53: Spectral Lines
  • each atom can produce a unique set of spectral lines
  • analysis of the spectral lines can determine composition
    • Slide # 54: Spectral Lines
  • Emission lines
  • spectral lines which are bright
  • Absorption lines
  • spectral lines which are dark
  • found only in front of a continuous spectra
    • Slide # 55: Spectral Lines
  • Absorption lines are dark
  • Emission lines are bright
    • Slide # 56: Lecture 9 : Walking the Planck
  • photoelectric effect
  • thermal radiation
  • Doppler shift
  • spectral lines
    • Slide # 57: Lecture 10: Atomic Physics for Kids of All Ages