Chapter 4 Radiation and Spectra

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Chapter 4Radiation and Spectra
The Sun in ultraviolet
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Radiation from Space ? Information from the Stars
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The Nature of Light

• At least 95 of the celestial information we
  receive is in the form of light.
• Astronomers have devised many techniques to
  decode as much of the encoded information as
  possible from the small amount of light that
  reaches Earth.
• This includes information about the object's
  temperature, motion, chemical composition, gas
  density, surface gravity, shape, structure, and
  more!

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The Nature of Light (contd)

• Most of the information in light is revealed by
  using spectroscopy
• Spectroscopy is the separation of light into its
  different constituent colors (or wavelengths) for
  analysis.
• The resulting components are called the spectrum
  of the light.

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Electric and Magnetic Fields

• Light is composed of electric fields and magnetic
  fields.
• Electric charges and magnets alter the region of
  space around them so that they can exert forces
  on distant objects.
• This altered space is called a force field (or
  just a field).

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Electromagnetism

• Connection between electric and magnetic fields
  was discovered in the 19th century.
• A moving electric charge or an electric current
  creates a magnetic field.
• Coils of wire are used to make the large
  electromagnets used in car junk yards or the tiny
  electromagnets in your telephone receiver.
• Electric motors used to start your car or spin a
  computer's hard disk around are other
  applications of this phenomenon.

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How it works

• A changing magnetic field creates electrical
  current---an electric field.
• Concept used in power generators---large coils of
  wire are made to turn in a magnetic field
• The coils of wire experience a changing magnetic
  field and electricity is produced.
• Computer disks and audio/video tapes encode
  information in magnetic patterns...
• When the magnetic disk or tape material passes by
  small coils of wire, electrical currents/fields
  are produced.

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James Clerk Maxwell

• Born/Educated in Scotland
• Lived 18311879
• Achieved a synthesis of knowledge of electricity
  and magnetism of his time.
• Hypothesis
• If a changing magnetic field can make an electric
  field, then a changing electric field should make
  a magnetic field.

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Electric and Magnetic Fields

• Consequence
• Changing electric and magnetic fields should
  trigger each other.
• The changing fields move at a speed equal to the
  speed of light.
• Maxwells conclusion.
• Light is an electromagnetic wave.
• Later experiments confirmed Maxwell's theory.

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The electric and magnetic fields oscillate at
right angles to each other and the combined wave
moves in a direction perpendicular to both of the
electric and magnetic field oscillations.
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Electromagnetic Radiation

• Light, electricity, and magnetism are
  manifestations of the same thing called
  electromagnetic radiation.
• Electromagnetic radiation is a form of energy.
• This energy exists in many forms not detectable
  by our eyes such as infrared (IR), radio, X-rays,
  ultraviolet (UV), and gamma rays.

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EM Waves General Properties

• Travels through empty space.
• Most other types of waves dont
• The speed of light (EM radiation) is constant in
  space.
• All forms of light have the same speed of 299,800
  kilometers/second in space
• This number is abbreviated as c.
• C f?, f c/?, ? c/f

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Wave Characteristics (1)

• Amplitude (A)
• A measure of the strength/size of the wave.
• Period (P)
• Duration of a cycle
• Units year, day, hours, seconds,
• Frequency (f)
• Rate of repetition of a periodic phenomenon.
• f1/P
• Units Hertz (Hz), or cycle/s.

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Period/Frequency Examples
Phenomenon Period Frequency
Earths orbit around the Sun 365 days 0.00274 days-1
Earths rotation 1 day or 86400 sec 1 day-1 or 1.16x10-5 Hz
Electrical Power (US) 0.0167 sec 60 Hz
Blue light 1.67x10-15 sec 6.0x1014 Hz
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Wave Characteristics (2)

• Wavelength (l)
• Size of one cycle of the wave in space.
• Units meter (m), centimeter (cm), micrometer
  (mm), nanometer (nm), angstrom (A).
• Velocity (v)
• Speed at which the wave propagate through space.
• v f x l
• Units m/s, miles/hour, km/hour, etc.

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Distance Units

• 1 meter 1 m
• 100 cm (centimeter)
• 1000 mm (millimeter)
• 1000000 mm (micrometer)
• 1000000000 nm (nanometer)
• 10000000000 Å (Angstrom)

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Visible Light
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Color Composition

• White light is made of different colors
  (wavelengths).
• White light passing through a prism or
  diffraction grating, is spread out into its
  different colors.

• First discovered by Newton

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Light Dispersion by refraction

• Refraction Angle or dispersion function of the
  wavelength (color)

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Max Plancks Photon

• Max Planck (lived 1858--1947)
• Discovered that if one considers light as
  packets of energy called photons, one can
  accurately explain the shape of continuous
  spectra.
• A photon is a particle of electromagnetic
  radiation.
• Bizarre though it may be, light is both a
  particle and a wave.
• Whether light behaves like a wave or like a
  particle depends on how the light is observed
• it depends on the experimental setup!

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Albert Einsteins Photon Energy Interpretation.

• Albert Einstein (lived 1879--1955)
• A few years after Planck's discovery Albert
  Einstein found a very simple relationship between
  the energy of a light wave (photon) and its
  frequency
• Energy of light h ? f (h ? c)/ l
• where h is a universal constant of nature called
  Planck's constant'' 6.63 ? 10-34 Jsec.

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Characterizing Light

• We now have three ways to characterize
  electromagnetic radiation
• wavelength
• frequency
• energy
• Astronomers use these interchangeably.
• We also divide the spectrum of all possible
  wavelengths/frequencies/energies into bands that
  have similar properties. Light is the most
  familiar of these.

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Visible Spectrum
Small wavelength High frequency High energy
large wavelength low frequency low energy
Remember the Spectrum ROY G BIV
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The Full Spectrum

• From the highest to lowest energy
• Gamma rays
• X-rays
• Ultraviolet
• Visible
• Infrared
• Microwave
• Radio

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EM Waves General Properties (contd)

• A wavelength of light is defined similarly to
  that of water waves
• distance between crests or between troughs.
• Visible light (what your eye detects)
• has wavelengths 400-800 nanometers. 1 nm 10-9
  m.
• Radio wavelengths are often measured in
  centimeters 1 centimeter 10-2 meter 0.01
  meter.
• The abbreviation used for wavelength is the Greek
  letter lambda l

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The Full E-M Spectrum
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EM Radiation Reaching Earth

• Not all wavelengths of light from space make it
  to the surface.
• Only long-wave UV, visible, parts of the IR and
  radio bands make it to surface.
• More IR reaches elevations above 9,000 feet (2765
  meters) elevation.
• That is one reason why modern observatories are
  built on top of very high mountains.

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Earths atmosphere is a shield

• Blocks gamma rays, X-rays, and most UV.
• Good for the preservation of life on the planet
• An obstacle for astronomers who study the sky in
  these bands.
• Blocks most of the IR and parts of the radio.
• Astronomers unable to detect these forms of
  energy from celestial objects from the ground
• Must resort to very expensive satellite
  observatories in orbit.

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Types of Spectra

• Continuous spectra consist of all frequencies
  (colors) - the thermal or blackbody spectrum is
  the most common example we will see.
• Absorption line spectra are continuous spectra
  with certain missing certain frequencies.
• Emission line spectra are a series of discrete
  frequencies (with or without a continuous
  spectra).
• Often astronomers deal with combinations of the
  above.

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Black Body Spectrum
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Star Color vs. Temperature
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Discrete Spectra

• Close examination of the spectra from the Sun and
  other stars reveals that the rainbow of colors
  has many dark lines in it, called absorption
  lines.
• They are produced by the cooler thin gas in the
  upper layers of the stars absorbing certain
  colors of light produced by the hotter dense
  lower layers.
• The spectra of hot, thin (low density) gas clouds
  are a series of bright lines called emission
  lines.
• In both of these types of spectra you see
  spectral features at certain, discrete
  wavelengths (or colors) and no where else.

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Absorption Line Spectrum
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Spectra

• The type of spectrum you see depends on the
  temperature of the thin gas.
• If the thin gas is cooler than the thermal source
  in the background, you see absorption lines.
• Since the spectra of stars show absorption lines,
  it tells you that the density and temperature of
  the upper layers of a star is lower than the
  deeper layers.
• In a few cases you can see emission lines on top
  of the thermal spectrum. This is produced by thin
  gas that is hotter than the thermal source in the
  background.

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Spectra (contd)

• The spectrum of a hydrogen-emission nebula
  (nebula'' gas or dust cloud) is just a series
  of emission lines without any thermal spectrum
  because there are no stars visible behind the hot
  nebula.
• Some objects produce spectra that are a
  combination of a thermal spectrum, emission
  lines, and absorption lines simultaneously!

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Spectra (contd)
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The Structure of the Atom
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Bohr atom

• Explanation for the discrete line spectra
• Niels Bohr (lived 1885--1962) provided the
  explanation in the early 20th century.
• Electrons only exist in certain energy levels and
  as long as an electron stays in a particular
  energy level, it doesnt emit any energy
  (photons).
• If an electron changes energy levels, it emits or
  absorbs energy in the form of a photon.
• Set of energies (light frequencies) uniquely
  identify the type of atom!

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Bohrs Model

• In Bohr's model of the atom, the massive but
  small positively-charged protons and massive but
  small neutral neutrons are found in the tiny
  nucleus.
• The small, light negatively-charged electrons
  move around the nucleus in certain specific
  orbits (energy levels).
• In a neutral atom the number of electrons the
  number of protons.
• The arrangement of an atom's energy levels
  (orbits) depends on the number of protons (and
  neutrons) in the nucleus and the number of
  electrons orbiting the nucleus.
• Because every type of atom has a unique
  arrangement of energy levels, they produce a
  unique pattern of absorption or emission lines.

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Isotopes
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How is light produced?
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Absorption Line Spectra
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Doppler Effect

• The wave nature of light means there will be a
  shift in the spectral lines of an object if it is
  moving.
• Sound Waves pitch ? frequency of wave
• Changes the pitch of the sound coming from
  something moving toward you or away from you
• train whistle, police siren
• Sounds from objects moving toward you are at a
  higher pitch because the sound waves are
  compressed together, shortening the wavelength of
  the sound waves.
• Sounds from objects moving away from you are at a
  lower pitch because the sound waves are stretched
  apart, lengthening the wavelength.

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Sound Waves

• Spread uniformly from a sound source
• Circles -- crests of the sound waves
• Think of waves spreading from a pebble dropped
  into a pool

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Doppler Shift
Moving towards Short wavelength
Moving away longer wavelength
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Doppler Shift ? Speed of Source
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Red shift, Blue shift

• Motion of the light source causes the spectral
  lines to shift positions.
• Which way the spectral lines shift tells you if
  the object is moving toward or away from you.
• Blue shift If the object is moving toward you,
  the waves are compressed, so their wavelength is
  shorter. The lines are shifted to shorter (bluer)
  wavelengths.
• Red shift If the object is moving away from you,
  the waves are stretched out, so their wavelength
  is longer. The lines are shifted to longer
  (redder) wavelengths.
• The doppler effect will not affect the overall
  color of an object unless it is moving at a
  significant fraction of the speed of light (VERY
  fast!)

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Doppler Shifted Spectra
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Doppler Shift (5)
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Doppler Shift (6)
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Expanding Universe

• The doppler effect tells us about the relative
  motion of stars with respect to us.
• The spectral lines of nearly all of the galaxies
  in the universe are shifted to the red end of the
  spectrum.
• These galaxies are moving away from our Milky Way
  galaxy.
• This is evidence for the expansion of the
  universe.