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Light

• Can act as a particle or a wave (wave-particle duality)
• Wavelength
• $\lambda$
• Distance between two successive crests
• Determines colour
• Frequency
• $f$
• Herz
• Cycles per unit of time
• Speed
• $c \; = \lambda f$
• Light travels at $3 x 10^8 m/s$

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 earlier/later than predicted at opposition/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