Light and Optics

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Light and Optics

• Newton experimented with prisms
• white light is composed of many colors a
  spectrum
• The earliest telescopes used lenses spread
  light like prism --- Newton designed a telescope
  that used mirrors
• What the lenses, or mirrors, do is collect and
  focus light

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• What do we mean by light
• Electromagnetic Radiation
• Created by changing electric and magnetic fields
• (James Maxwell)
• Electromagnetic Radiation moves at the speed of
  light in a vacuum (300,000 km per second)
• Light is a wavelike phenomenon that is,
  periodic
• Characterized by wavelength (?) and frequency (f)
• Such that ? c / f
• Wavelength has units of length frequency has
  units of 1/time (i.e. per second)

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• Light cannot be completely described as a wave
  Newton suggested that light was made up of
  particles photons
• A photon carries energy inversely proportional to
  its wavelength
• Energy h c / ? h f
• (h is Plancks constant)

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• Gamma rays have the highest energy
• Radio waves have the lowest energy
• What can we detect on Earth (that is, what
  penetrates our atmosphere)? Visible light, short
  ? Infrared, some radio waves for the rest, need
  to observe from space

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Telescopes

• Refracting, uses lenses to collect, focus light
• Problems 1) Chromatic Aberration, short ? light
  is bent more than long ? need an achromatic
  lens to bring any two colors to the same focus
• 2) size, weight, expense
• Reflecting, uses mirrors
• Newtonian, Cassegrain, Schmidt-Cassegrain
• Mountings Equatorial (siderial drive)
  Alt-Azimuth (computer controlled drive)

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• Why use telescopes?
• Light Gathering Power (detect fainter features,
  objects, etc.)
• LGPa LGPb (Da Db)2
• D is the diameter of the objective lens or mirror
• Resolving Power diffraction fringes, due to
  wavelike nature of light
• Bigger objective, smaller fringes
• Such that a (seconds of arc) 11.6 / D (in cm)
• Seeing due to atmosphere, limits resolution
  to 0.5 seconds of arc need Adaptive optics.

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• Magnifying Power M Fob / Fep (not usually
  important for science
• Light Pollution a major problem. Observatories
  must be located far away from cities (typically
  on mountain tops)
• BIG MIRRORS why? Light gathering and resolving
  power maximized
• Cost, weight limitations
• Biggest? 6-meter in Crimea
• Biggest single mirror? 5-meter on mtg. Palomar
• An 8-meter diameter mirror would be too heavy.
• Solution? Segmented Mirrors 10-meter Keck I and
  II
• Thin mirrors sag need active optics (New
  Technology Telescope, Multi-Mirror Telescope,
  Very Large Telescope, Gemini Project

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How Data are Recorded

• First images and spectra used photographic plates
  (in 19th Century until recently)
• Photometers electronic measurement of intensity
  
• Charge-Coupled Devices CCDs typically a chip
  with 106 light detectors
• -- cheap ones used in digital cameras
• -- used in the cameras on the Hubble
• CCDs are much more sensitive than photographic
  plates after taking the image, the chip isread
  out

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Spectrographs

• Optical apparatus to generate a spectrum
• Can be a prism, or, more usually, a grating
• Light is dispersed, producing a spectrum
• A comparison spectrum, from a known element, is
  taken to calibrate the wavelengths

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Radio Telescopes

• Can detect cold gas
• Radio waves can penetrate cosmic dust
• Some objects emit most of their energy as radio
  waves
• Consist of a reflecting dish, antenna, amplifier,
  recorder
• Limitations resolving power (depends on ?), low
  intensity (low photon energy), interference
• To maximize resolution radio interferometry
• To overcome low intensity Big dishes

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Infrared Astronomy

• window 1,200 40,000 nanometers
• Parts of this range blocked by O2, CO2, H2O
• Far IR comets, planets, star formation
• Need to get above the Earths atmosphere
• (balloons, airplanes)
• Space Based IRAS, ISO, SIRTF

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Higher Energies

• Ultra Violet 320 115 nanometers
• IUE (1978), EUVE, FUSE
• Hot stars, hot gas
• X-ray, 10 0.1 nanometers
• Very hot gas, violent events
• Chandra X-ray Observatory, Newton-XMM
• Gamma-ray exploding stars, neutron stars, black
  holes
• Compton Gamma Ray Observatory

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• Hubble Space Telescope (launched 1990)
• 2.4 meter mirror (not big by standards of
  ground-based telescopes
• But, can detect 50 times fainter objects, 10
  times smaller details. Why?
• Observes in range from UV to IR
• Several cameras and spectrographs

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• Other than E-M radiation?
• Cosmic Rays, neutrinos
• Meteorites, other rocks/dust

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Light and Atoms

• Nearly all our information from space comes in
  the form of E-M radiation
• Fraunhofer discovered 600 dark lines in the
  spectrum of the Sun
• These are due to different elements in the Suns
  atmosphere
• Stars are different colors the Sun appears
  yellow Why?

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What is Temperature?

• Measure of the average motion of particles in an
  object
• Higher temperature means faster motion
• Thermal Energy measure of the total energy
  stored in the motion of particles in a body
• in a hot gas, atoms move faster than in a colder
  gas

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• Photons produced by changing electric fields.
  When electrons (subatomic, charged particles) are
  accelerated, E-M radiation is emitted
• When atoms collide with electrons in a hot gas,
  energy is carried away by photons
• Black Body Radiation
• The hotter an object, more radiation emitted
• There is a wavelength, ?max, at which the peak
  amount of radiation is emitted
• Stefan Boltzmann Law E s T4
• Wiens Law ?max (nanometers) 3 x 106 /
  T(kelvin)
• So, blue stars are hotter than red stars, and
  give off more energy

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The Model Atom

• Bohr model there is a nucleus, in which there
  are protons (positively charged) and neutrons
  (neutral charge).
• Orbiting the nucleus are negatively charged
  particles electrons
• The electron is bound to the atom by the Coulomb
  Force
• Mass of a proton 1.67 x 10-27 kg
• Diameter of a proton 1.6 x 10-15 meters
• Size of the electron cloud 4 x 10-10 meters
• Different chemical elements different numbers
  of protons in the nuclei (H 1 proton, He 2
  protons, C 6, etc.)
• Numbers of neutrons can vary different isotopes

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• Ionization to remove an electron from an atom
  results in an ion and a free electron
• Recombination ion electron ? neutral atom (or,
  more neutral, anyway)
• Since electrons bound to atom by Coulomb force,
  need to add energy to ionize (Binding Energy)
• The further away the electron is from the
  nucleus, the less the binding energy.
• In the Model Atom, only certain orbits are
  permitted (quantum mechanics) these permitted
  orbits are different for each atom or ion.
• The lowest orbit (or level) is the most tightly
  bound also called the ground state
• Higher levels are called excited states energy
  is required to get the electron into an excited
  state. The energy can come via
• Collisions
• Photons
• Need an exact amount of energy in the photon
• After excitation, the electron will fall back to
  the ground state, and emit photons in the process

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Three Types of Specta

• Continuous Black Body spectrum is one example
• Emission Line (or bright line)
• Absorption Line (or dark line)
• Kirchoffs Laws
• I Continuous spectra a solid, liquid, or
  dense gas, excited to emit light, will radiate at
  all wavelengths, producing a continuous spectrum
• II Emission Line a low-density gas, excited
  to emit light, will do so at specific
  wavelengths, producing an emission line spectrum
• III Absorption Line If light comprising a
  continuous spectrum passes through a cool,
  low-density gas, the resulting spectrum will have
  dark lines at certain wavelengths, hence an
  absorption line spectrum.