Visible Light

Speed of Light

  • 3×105km/s3 \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