Lecture #6: Thursday, Jan. 26

  1. Review
    1. Angular measurements (picture)
    2. Angular size of the moon is about the same as of the sun (0.5 degree)
    3. Celestial sphere coordinates: Right Ascension (RA) and Declination (Dec)
      RA: angle along equator staring eastward from vernal equinox (24 hours = 360 degree)
      Dec: angle away from equator - towards north: positive, south: negative
    4. Length of a solar day: 24 hours
      Length of a siderial day: 4 min less because of motion of the sun around earth
    5. Origin of Seasons: Light in summer is more pependicular ("concentrated") and days are longer
         than in winter
    6. Origin of Phases of the Moon: Motion of moon around the earth
    7. Origin of Tides: Moon tries to deform Earth
  2. Eclipses

  1. Solar eclipse: Sun and moon have roughly the same angular size, therefore
    the moon can sometimes cover the sun. The earth is then in the shadow of the moon (picture)
    Phase of the moon ?
    DEMO

  2. The orbit of the moon is inclined by 5 degree to the ecliptic. There is not an eclipse every month.

  3. Lunar eclipse: The moon is in the shadow of the earth. (same picture)
    Phase of the moon ?

  1. The Nature of Light
    1. Light has wave and particle properties
    2. Wave Basics
      (DEMO: hose, wavegenerator - wavelength, amplitude, speed)
      (DEMO: frequency)
  1. Wavelength (l)    : Distance from crest to crest
  2. Amplitude              Height of the crest (from baseline)
  3. Frequency:             Number of crests that passes an observer per second
    Unit: Hertz (Hz)  1 Hz = 1 crest / second
    Period and Frequency (f) (Frequency = 1/ period )
  4. Wave speed           Speed of a wave crest
  5. Wave Speed = wavelength times frequency (c=lf)

    or in other words:
    frequency     = wave speed divided by wavelength (f=c/l)
  1. Light as a Wave
    1. Light is an electromagnetic wave. Electric and magnetic fields wave 
      perpendicular to each other. Light propagates "itself" and needs no medium. 
      (good for astronomy - it goes through vacuum)
    2. Wave speed: speed of light. Vacuum: 300,000 km/s. In medium slower !
      (velocity = velocity in vacuum / index of refraction)
      "Speed limit of the Universe" 
    3. Amplitude squared = Intensity
    4. Frequency or wavelength = color
      red        : 700 nm   (frequency = c/700nm = 4.2e14 Hz)
      yellow   : 580 nm
      green     : 530 nm
      blue       : 450 nm

      1 nm = 1 nano meter = 1e-9m or a millionth of a millimeter.
  2. Generation of light 
    1. Waves are created if something moves up and down
    2. Light is created if a charged particle moves up and down
      for example the electrons in matter
    3. What makes them move ? For example heat. 
      At T=0 K (= -273 degree C = -459 degree F) no motion ("absolute zero")
      room temperature: 293K (20 degree C, 68 degree F): electrons move !

      degree C = 5/9 * (degree F - 32)

      (DEMO: filament)
    4. Why do stars emit light ? Because they are HOT !
    5. Hotter means brighter
      Stefan-Boltzmann Law:     Intensity = size * temperature4
  3. Light as a particle
    1. Sometimes, light behaves more like a particle stream
    2. Light comes in small energy packets called "Photons"
    3. Intensity = number of photons per second
    4. Frequency = energy of the photons
      E = h * f   with h as Planck constant: 4.136E-15 eV/s

      (measure photon energy in eV (1 eV = 1.6 x 10-19 Joule))

      (Joule is the SI unit for Energy: 1 Joule/s = 1 Watt, so for example a 100 Watt 
      light bulb emits 100 Joule/s)

      use frequency = c / wavelength to get handy formula:
      E (in eV) = 1240/wavelength in nm

      (DEMO: photons in the classroom)

  1. Electromagnetic Spectrum
    1. There are many more colors than we can see !!!
      We don't have "color" names for them, but we name them according to 
      their frequency or wavelength range and what they do for us:

      higher frequency, shorter wavelength than visible light are:
      • Ultraviolet (400nm - 20 nm)
      • X-rays      (20 nm - 1/100 nm)
      • Gamma rays (shorter than that)

      lower frequency, longer wavelength than visible light are:

      • Infrared  ( 0.001 mm  - 0.1 mm)
      • Microwaves ( 0.1mm - 10 cm)
      • Short wave radio waves ( 10cm - 10m)
      • Long wave radio waves  (longer than 10m)
    2. Eyes response to electromagnetic radiation:
      (picture)
      We can see photons from of 1.8 eV (700nm, red) to 3.1 eV (400 nm violet)
      Can we see other colors ? Not with the eyes, but some we can feel
      (DEMO: Infrared Lamp)
    3. Opacity of the Atmosphere
      (picture)
      1. Penetrates easily:
        • visible light (except during winter in East Lansing)
        • radio waves (2cm - 10m)
        • Some infrared wavelengths
      2. Penetrates somewhat:
        • Ultraviolet (mostly blocked by Ozone layer)
      3. All other radiation is blocked
  2. Interference
    1. Two waves added can interfere:
      1.  constructively - the result is a stronger wave
      2. destructively - the result is a weaker wave (or no wave at all)

      DEMO: Interference basics, Laser Light through slits, 2 meter sticks

    2. Interference of light produces an Interference patternI
    3. nterference pattern contains information about the size of the source.
      This is used in astronomy where sometimes the light from several telescopes
      is added together and from the interference pattern one can resolve finer structures
      as would be possible from a single telescope:
      Example: Very Large Telescope (optical) or Very Large Array (Radio)