Chapter 3: Light and Atoms cont'

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Chapter 3 Light and Atoms cont.

• Dr. Michelle Arnold
• September 16, 2009

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Light as a Wave

• Light is an electromagnetic wave (changing
  electric and magnetic field)
• Light does not require a medium to travel
• Speed of light, c 300 000 000 m/s (fast!)

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• Wave properties
• Wavelength (?) distance between crests
• Unit Meter (m), nm 10-9 m
• Frequency (f) number of crests that pass per
  second
• Unit Hertz (Hz), crests per second
• Speed (c)
• Units m/s, c 300 000 000 m/s for light
• c ?f
• All light travels at the same speed (c)
• longer wavelengths have smaller frequencies
• shorter wavelengths have greater frequencies

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• Electromagnetic Spectrum
• Visible Light wavelength of 400 (violet) 700
  (red) nm
• White light is a mixture of ALL the visible light
  colors

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• Note Only visible light, radio waves and some
  infrared can penetrate the Earths atmosphere and
  be seen by telescopes at the surface of the
  planet
• Telescopes in orbit (such as the Hubble Space
  Telescope) can see all parts of the
  electromagnetic spectrum

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Light as a Particle

• When thinking about the energy carried by light,
  it works to think of light as a stream of
  particles called photons
• Every color of light has photons with different
  energy
• Higher frequency/smaller wavelength light has
  greater energy photons
• E hc/? where h is Plancks constant

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Wave-Particle Duality

• Light is a wave and a particle???
• All particles (electrons, protons, etc) have both
  wave and particle properties
• We will mainly deal with the wave nature of light

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Aside Temperature Scales

• Temperature is a measure of how fast the atoms in
  an object are moving
• Fahrenheit
• Water freezes at 32oF
• Water boils at 212oF
• Celsius
• Water freezes at 0oC
• Water boils at 100oC
• Kelvin
• 0 K is absolute zero, when all atomic motion
  stops
• Kept 100 degrees between water freezing and
  boiling
• 0 K -273oC

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Any Object Not at Absolute Zero Radiates Light!

• We are not talking about reflected light, but
  light emitted by the object itself
• An object will emit a range of light
• Hotter objects emit
• the greatest intensity at smaller wavelengths
  (?m) than cooler objects
• More light in total than cooler objects
• Looking at a stars spectrum will tell us the
  temperature of the star!

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