Light and Atoms

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Light and Atoms
Chapter Three
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• No laboratory jar on Earth holds a sample labeled
  Star Stuff, and no instrument has ever probed
  inside a star
• The stars are far beyond human reach, and the
  only information you can obtain about them comes
  hidden in light

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• Due to the vast distances, with few exceptions,
  direct measurements of astronomical bodies are
  not possible
• If we want to know how hot the sun is, we cant
  just stick a thermometer in it
• When light reaches our atmosphere, gases there
  alter its properties, blocking some rays, bending
  and blurring others

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• Light from a distant star or planet can tell us
  what the body is made of, its temperature, and
  other properties
• So light is our key to studying the universe

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• Light is a form of energy that can be thought of
  as either a wave or as a stream of particles
• Light is just a part of the radiation emitted by
  astronomical objects
• Light can be produced within an atom by changes
  in its electrons energies
• LIGHT SIGNATURE

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3.1

• Light is radiant energy it does not require a
  medium for travel
• Light travels at 299,792.458 km/s in a vacuum
  (fast enough to circle the Earth 7.5 times in one
  second)
• However, the speed of light is reduced as it goes
  through transparent materials and the speed is
  also dependent on color

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• The Nature of Light Waves or Particles?
• One model of light
• electromagnetic wave
• The wave travels as a result of a fundamental
  relationship between electricity and magnetism

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• A changing magnetic field creates an electric
  field and a changing electric field creates a
  magnetic field
• Wave picture cannot explain all of lights
  properties

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• Another model of light photons
• Light thought of as a stream of particles
• Each photon particle carries energy

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• Wave Particle Duality
• In a vacuum, photons travel in straight lines,
  but behave like waves
• Sub-atomic particles also act as waves
• Wave-particle duality All particles of nature
  behave as both a wave and a particle
• Which property of light manifests itself depends
  on the situation

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• Colors to which the human eye is sensitive is
  referred to as the visible spectrum

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• In the wave theory, color is determined by the
  lights wavelength (symbolized as l)
• Nanometer (10-9 m) is the convenient unit
• Red 700 nm (longest visible wavelength), violet
  400 nm (shortest visible wavelength)

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• We Characterize Electromagnetic Waves by Their
  Frequency
• Frequency (or n) is the number of wave crests
  that pass a given point in 1 second
• (measured in Hertz, Hz)
• Important relationship nl c
• frequency x wavelength light

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• Light with no distinguishing color is called
  white light

is a mixture of all colors
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3.2

• Electromagnetic spectrum is composed of gamma
  rays, x-rays, ultraviolet rays, visible light,
  infrared rays, radio waves
• Various instruments used to explore the various
  regions of the spectrum

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• Infrared Radiation
• Sir William Herschel (around 1800) showed heat
  radiation related to visible light
• He measured an elevated temperature just off the
  red end of a solar spectrum infrared energy
• Our skin feels infrared as heat
• Longer wavelengths than visible light (cant see)

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• Ultraviolet Light
• J. Ritter in 1801 noticed silver chloride
  blackened when exposed to light just beyond the
  violet end of the visible spectrum
• Shorter wavelengths than visible light (cant see)

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• Radio Waves
• Predicted by Maxwell in mid-1800s, Hertz produced
  radio waves in 1888
• Jansky discovered radio waves from cosmic sources
  in the 1930s, the birth of radio astronomy
• Radio waves are used to study a wide range of
  astronomical processes
• Used for communication, microwave ovens, and
  search for extraterrestrials

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• X-rays
• Roentgen discovered X rays in 1895
• First detected beyond the Earth in the Sun in
  late 1940s
• Used by doctors to scan bones and organs
• Used by astronomers to detect black holes and
  tenuous gas in distant galaxies

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• Two parts of the electromagnetic spectrum remain
  relatively unexplored
• The region between infrared radio waves
• Gamma rays
• difficult to measure because they fall in
  wavelength bands that are strongly blocked by
  earths atmosphere

Gamma
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• The warmth we feel on our face from a beam of
  sunlight demonstrates that light carries energy,
  but not all wavelengths carry the same amount of
  energy
• short wavelength radiation carries more energy
  than a long wavelength radiation
• ultraviolet light with its short wavelength
  carries proportionally more energy than infrared
  light with its longer wavelength

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• Wiens Law the wavelength at which a body
  radiates most strongly is inversely proportional
  to the bodies temperature
• As a body gets hotter, it emits more light at
  shorter wavelengths
• when you first turn on an electric stove it glows
  dull red, but as it heats up the glow becomes
  bright orange and eventually yellow

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3.3

• Structure of Atoms
• Nucleus Composed of densely packed (neutral
  charged) neutrons and positively charged protons
• A cloud of negative electrons are held in orbit
  around nucleus by positive charge of protons
• The electron orbits are quantized, can only have
  discrete values and nothing in between

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• An element is a substance composed of only one
  type of atom
• A neutral element will contain an equal number of
  protons and electrons
• The chemical properties of an element are
  determined by the number of electrons

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3.4

• Energy Change in an Atom
• An atoms energy is increased if an electron is
  lifted to an outer orbit the atom is said to be
  excitedABSORPTION
• An atoms energy is decreased if an electron
  moves to an inner orbitEMISSION

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3.5

• The key to determining the composition and
  conditions of an astronomical body is analysis of
  its spectrum
• Spectroscopy is the technique to capture and
  analyze a spectrum produced by atoms interacting
  with light
• Spectroscopy assumes that every atom or molecule
  will have a unique spectral signature

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• How a Spectrum is Formed
• Electron orbits are more properly thought of as
  energy levels with the lowest energy level
  corresponding to the smallest orbit
• The wavelength of emitted (or absorbed) light is
  calculated from the energy difference of the two
  levels involved

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• Atoms in the gas absorb only those wavelengths
  whose energy equals the energy difference between
  their electron orbits
• The lost light makes the spectrum darker at the
  wavelengths where it is absorbed

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• There are different types of Spectra
• Continuous spectrum
• Intensity of the radiation changes smoothly from
  one wavelength to the next
• Typical objects are solids and dense gases

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• Emissionline spectrum
• Light is emitted at only a few particular
  wavelengths while others stay dark
• Examples include fluorescent tubes, aurora, and
  many interstellar clouds
• Produced by hot tenuous (unstable) gases

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• Dark-line or absorptionline spectrum
• Light passes through cooler gas leaving dark
  absorption lines
• First discovered in 1802 by William Wollaston
  when he viewed sunlight through a prism and a
  narrow slit
• Joseph Franhofer later categorized these lines
  when he discovered similar ones in other stars

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3.6

• Doppler Shift is the change in the observed
  wavelengths of radiation caused by the motion
  of the emitting body or the observer
• There is an increase in the wavelength if the
  source and observer move apart
• There is a decrease in wavelength if the source
  and the object approach each other

Ambulance siren pitch drops as it drives away
from you
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• An observed increase in wavelength is called a
  redshift
• A decrease in observed wavelength is called a
  blueshift

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3.7

• Gases in the Earths atmosphere absorb
  electromagnetic radiation to the extent that most
  wavelengths from space do not reach the ground
• Visible light, most radio waves, and some
  infrared penetrate the atmosphere through
  atmospheric windows, wavelength regions of high
  transparency
• Lack of atmospheric windows at other wavelengths
  is the reason for astronomers placing telescopes
  in space

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Gamma