Light

• Can act as a particle or a wave (wave-particle duality)
• Wavelength
  • Determines colour
• Frequency
  • Cycles per unit of time
• Speed
  • $c ; = \lambda f$
  • Light travels at $3 x 10^8 m/s$
• Light is additive
• Used to make observations in astronomy

Visible Light

• Part of electromagnetic spectrum to which our eyes are sensitive
• What humans can see
• ROYGBIV
  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet
• Large wavelength (R) --> small wavelength (V)
• Small frequency (R) --> large frequency (V)

Speed of Light

• $3 \times 10^5 km/s$
• First sign that light's speed was finite came in 1675
  • Ole Rømer observed eclipses of Jupiter's moons
    • Noticed the eclipses happened at times different than predicted
      • Earlier than predicted at opposition
      • Later than predicted at conjunction
    • Since the distance between opposition/conjunction is different, speed of light must be have an impact
• Takes ~8.3 minutes for light to get from the Sun to Earth
  • 1 AU --> takes light 8 minutes to travel

Physics