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1ISP 205 - Astronomy Gary D. Westfall Lecture 6
The Nature of LightThe Nature of Light• Light and other forms of radiation carry
information to us from distance astronomical objects
• Visible light is a subset of a huge spectrum of electromagnetic radiation
• Maxwell pioneered the theory of electromagnetic radiation (and light) Electric fields Magnetic fields Oscillating charges produce electric and magnetic
fields Famous 4 equations (outside the scope of this course)
2ISP 205 - Astronomy Gary D. Westfall Lecture 6
Light as a WaveLight as a Wave• Wave
Diffraction Interference
• Can describe waves in terms of wavelength and frequency
WavelenthCrestTroughMoving at the speed of light
3ISP 205 - Astronomy Gary D. Westfall Lecture 6
Electromagnetic RadiationElectromagnetic Radiation• Light differs from other forms of electromagnetic
radiation by its wavelength• Visible light has wavelengths between 400 and
700 nanometers• EM radiation with wavelengths just longer than
visible light is called infrared radiation (heat)• EM radiation with wavelength just shorter than
visible light is called ultraviolet radiation (UV)• Radio waves have long wavelengths (WKAR FM
is 3 meters)• Microwaves have about 3 cm wavelength
4ISP 205 - Astronomy Gary D. Westfall Lecture 6
EM Radiation SpectrumEM Radiation Spectrum• The frequency/wavelength varies dramatically
• Most EM radiation cannot penetrate the Earth’s atmosphere
5ISP 205 - Astronomy Gary D. Westfall Lecture 6
View of the Sky with X raysView of the Sky with X rays• If we could “see” with X rays instead of visible
light and we were above the Earth’s atmosphere the sky would look like:
6ISP 205 - Astronomy Gary D. Westfall Lecture 6
Light as a ParticleLight as a Particle• Light (and all EM radiation) exists in quantized
units called photons
• A photon carries a specific amount of energy
• High frequency EM radiation has high energy photons Gamma rays
• Low frequency EM radiation has low energy photons Long-wave radio
• Described by Quantum Mechanics
7ISP 205 - Astronomy Gary D. Westfall Lecture 6
Radiation and TemperatureRadiation and Temperature• The temperature
of an object determines what wavelength of EM radiation it will emit
• The wavelength of the maximum energy emission is given by Wien’s LawmaxT = 2.9 x 10-3 mK
8ISP 205 - Astronomy Gary D. Westfall Lecture 6
Energy Emitted by StarsEnergy Emitted by Stars• The higher the temperature of an object, the more
energy is radiated at all wavelengths
• The higher the temperature, the “bluer” the star looks
• The total energy radiated is given by the Stefan-Boltzmann law E = T4 where E is the emitted energy, T is the
temperature, and is a constant
9ISP 205 - Astronomy Gary D. Westfall Lecture 6
Spectroscopy in AstronomySpectroscopy in Astronomy• EM radiation carries information about the nature
of astronomical object• Visible light is the most used• Light can be
Reflected From a mirror
Refracted Through a lens
Dispersed Separated by wavelength
Prism Spectrometer
10ISP 205 - Astronomy Gary D. Westfall Lecture 6
Continuous SpectrumContinuous Spectrum• When white light (a superposition of light with all
wavelengths) is dispersed with a prism or a spectrometer, all colors (wavelengths) are visible
• Wavelengths shorter than 400 nm are invisible (UV)
• Wavlengths longer than 700 nm are invisible (IR)
11ISP 205 - Astronomy Gary D. Westfall Lecture 6
Discrete Emission SpectraDiscrete Emission Spectra• When atoms are heated, they emit light at specific
wavelengths characteristic of those atoms
12ISP 205 - Astronomy Gary D. Westfall Lecture 6
Discrete Absorption SpectraDiscrete Absorption Spectra• When white light passes through atoms light is
absorbed at specific wavelengths
• Several elements were first observed in absorption spectra from the sun
13ISP 205 - Astronomy Gary D. Westfall Lecture 6
Probing the AtomProbing the Atom• The electron was discovered by J.J. Thomson in 1897
Related to electricity, lightning
• In 1911, Ernest Rutherford bombarded a thin foil of gold with alpha particles from naturally occurring radioactive radium
14ISP 205 - Astronomy Gary D. Westfall Lecture 6
Rutherford’s Model of the AtomRutherford’s Model of the Atom• Rutherford’s results showed that most of the mass
of the atom was concentrated in the nucleus
• Rutherford proposed a model similar to the solar system with negative electrons orbiting a positive nucleus
15ISP 205 - Astronomy Gary D. Westfall Lecture 6
The Hydrogen AtomThe Hydrogen Atom• The simplest atom is the hydrogen atom• Composed of 1 electron and 1 proton
Electron has charge -1 Proton has charge +1 Proton is 2000 times heavier
• The electron is bound to the proton in its ground state
• We know now that the electron does not orbit the proton like the Earth orbits the Sun
Heisenberg Uncertainty Principle We cannot simultaneously know the position and energy of a particle to
arbitrary precision
16ISP 205 - Astronomy Gary D. Westfall Lecture 6
Other AtomsOther Atoms• The next most simple atom is
helium
• A helium atom has 2 neutrons and 2 protons in its nucleus with 2 electrons orbiting the nucleus The neutron and proton have almost the same mass but
the neutron has not charge Neutron not discovered until 1930 by Chadwick
• The helium atom is much more complicated than the hydrogen atom because the 2 electrons interact with each other
17ISP 205 - Astronomy Gary D. Westfall Lecture 6
IsotopesIsotopes• The chemical properties of atoms are determined by the number
protons and the number of electrons
• Light nuclei have roughly the same number of neutrons and protons
• Atomic nuclei can have different number of neutrons Isotopes
• Hydrogen has 3 naturally occurring isotopes
Hydrogen, 1H Stable
Deuterium, 2H Stable
Tritium, 3H Radioactive
18ISP 205 - Astronomy Gary D. Westfall Lecture 6
The Bohr AtomThe Bohr Atom• Rutherford’s model of the atom had some tragic
flaws
• Orbiting electrons are accelerating and should radiate energy Lifetime of the atom should be 10-10 seconds!
• Neils Bohr proposed that the electrons in the hydrogen atom could only exist in certain quantized orbits
• Jumping between the orbits required the emission or absorption of photons of a specific wavelength
19ISP 205 - Astronomy Gary D. Westfall Lecture 6
Radiation and AbsorptionRadiation and Absorption• Whenever a hydrogen atom changes from one stationary state to another, energy
is emitted or absorbed. When that energy takes the form of electromagnetic radiation then it has a frequency f (or as it is often called, ) given by h = |Ef - Ei|
Ef and Ei are the final and initial energies respectively.
• If Ei > Ef, then radiation occurs while if Ef > Ei, then absorption takes place.
• In the diagram the first six levels are shown as well as the zero energy level (n = ).
• If the transition takes place from any n to n =1, it is referred to as a Lyman line.
• Transitions to n = 2 are called Balmer lines and so on.
• Four of the Balmer lines are in the visible range.
20ISP 205 - Astronomy Gary D. Westfall Lecture 6
Photon EnergiesPhoton Energies• Visible light has wavelengths between 400 and
700 nm
• Photons have energy E = hf = hc/• Photons from visible light then have energies
between 700 nm
E = 6.62 x 10-34 * 3 x 108 / 700 x 10-9 = 2.8 x 10-19 J = 1.8 eV 400 nm
E = 6.62 x 10-34 * 3 x 108 / 400 x 10-9 = 5.0 x 10-19 J = 3.1 eV
21ISP 205 - Astronomy Gary D. Westfall Lecture 6
Three Kinds of SpectraThree Kinds of Spectra• We will consider three kinds of spectra
Continuous Light bulb or other source
Emission Heated cloud of gas
Absorption Continuous spectra passing through a cold cloud of gas
22ISP 205 - Astronomy Gary D. Westfall Lecture 6
Doppler ShiftDoppler Shift• Relative motion affects
waves
• If a source of waves is moving toward you, the frequency is higher and the wavelength is shorter
• If a source of waves is moving away from you the frequency is lower and the wavelength is longer
• Toward, shorter wavelength, blue shift
• Away, longer wavelength, red shift
• A familiar example is sound
23ISP 205 - Astronomy Gary D. Westfall Lecture 6
Red ShiftRed Shift• Most of the objects in the universe seem to be
moving away from us Evidence for Big Bang Red shift v = c/
• We observe the red shift of specific emission lines from known atoms Hydrogen or calcium have distinctive lines and are
almost always present
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