Light%20hits%20Matter:%20Refraction

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Light hits Matter Refraction

• Light travels at different speeds in vacuum, air,
  and other substances
• When light hits the material at an angle, part of
  it slows down while the rest continues at the
  original speed results in a change of direction
• Different colors bend different amounts prism,
  rainbow

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Application for Refraction

• Lenses use refraction to focus light to a single
  spot

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Light hits Matter (II) Reflection

• Light that hits a mirror is reflected at the same
  angle it was incident from
• Proper design of a mirror (the shape of a
  parabola) can focus all rays incident on the
  mirror to a single place

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Application for Reflection

• Curved mirrors use reflection to focus light to a
  single spot

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Telescopes

• From Galileo to Hubble Telescopes use lenses and
  mirrors to focus and therefore collect light

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Rain analogy Collect light as you collect rain

• Rain/light collected is proportional to area of
  umbrella/mirror, not its diameter

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Telescopes

• Light collectors
• Two types
• Reflectors
• (Mirrors)
• Refractors (Lenses)

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Refracting Telescopes
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Reflecting Telescope
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Problems with Refractors

• Different colors (wavelengths) bent by different
  amounts chromatic aberration
• Other forms of aberration
• Deform under their own weight
• Absorption of light
• Have two surfaces that must be optically perfect

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Telescope Size

• A larger telescope gathers more light (more
  collecting area)
• Angular resolution is limited by diffraction of
  light waves this also improves with larger
  telescope size

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Resolving Power of Telescopes
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Atmospheric Limitations
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Light From gamma-rays to radio waves

• The vast majority of information we have about
  astronomical objects comes from light they either
  emit or reflect
• Here, light stands for all sorts of
  electromagnetic radiation
• A type of wave, electromagnetic in origin
• Understanding the properties of light allows us
  to use it to determine the
• temperature
• chemical composition
• (radial) velocity
• of distant objects

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Waves

• Light is a type of wave
• Other common examples ocean waves, sound
• A disturbance in a medium (water, air, etc.) that
  propagates
• Typically the medium itself does not move much

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Wave Characteristics

• Wave frequency how often a crest washes over you
• Wave speed wavelength (?) ? frequency (f)

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Electromagnetic Waves

• Medium electric and magnetic field
• Speed 3 ?105 km/sec

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Electromagnetic Spectrum
Energy low ? medium ?
high
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Electromagnetic Radiation Quick Facts

• There are different types of EM radiation,
  visible light is just one of them
• EM waves can travel in vacuum, no medium needed
• The speed of EM radiation c is the same for all
  types and very high (? light travels to the moon
  in 1 sec.)
• The higher the frequency, the smaller the
  wavelength (?? f c)
• The higher the frequency, the higher the energy
  of EM radiation (E h f, where h is a constant)

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Visible Light

• Color of light determined by its wavelength
• White light is a mixture of all colors
• Can separate individual colors with a prism

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Three Things Light Tells Us

• Temperature
• from black body spectrum
• Chemical composition
• from spectral lines
• Radial velocity
• from Doppler shift

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Temperature Scales
Fahrenheit Centigrade Kelvin
Absolute zero ?459 ºF ?273 ºC 0 K
Ice melts 32 ºF 0 ºC 273 K
Human body temperature 98.6 ºF 37 ºC 310 K
Water boils 212 ºF 100 ºC 373 K
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Black Body Spectrum

• Objects emit radiation of all frequencies, but
  with different intensities

Ipeak
Higher Temp.
Ipeak
Ipeak
Lower Temp.
fpeakltfpeak ltfpeak
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Cool, invisible galactic gas (60 K, fpeak in
low radio frequencies)
Dim, young star (600K, fpeak in infrared)
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The Suns surface (6000K, fpeak in visible)
Hot stars in Omega Centauri (60,000K, fpeak in
ultraviolet)
The higher the temperature of an object, the
higher its Ipeak and fpeak
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Wiens Law

• The peak of the intensity curve will move with
  temperature, this is Wiens law
• Temperature wavelength constant
• 0.0029 Km
• So the higher the temperature T, the smaller
  the wavelength, i.e. the higher the energy of the
  electromagnetic wave

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Example

• Peak wavelength of the Sun is 500nm, so
• T (0.0029 Km)/(5 x 10-7 m) 5800 K
• Instructor temperature roughly 100 F 37C
  310 K, so
• wavelength (0.0029Km)/310 K
• 9.35 10-6 m
• 9350 nm ? infrared radiation
• 10 µm 0.01 mm

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Measuring Temperatures

• Find maximal intensity
• ? Temperature (Wiens law)

Identify spectral lines of ionized elements ?
Temperature
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Color of a radiating blackbody as a function of
temperature

• Think of heating an iron bar in the fire red
  glowing to white to bluish glowing