Light and the Electromagnetic Spectrum

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Light and the Electromagnetic Spectrum
2
Light Phenomenon

• Isaac Newton (1642-1727) believed light consisted
  of particles
• By 1900 most scientists believed that light
  behaved as a wave.

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The Electromagnetic Spectrum

• The electromagnetic spectrum represents the
  range of energy from low energy, low frequency
  radio waves with long wavelengths up to high
  energy, high frequency gamma waves with small
  wavelengths.

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• Visible light is a small portion of this
  spectrum. This is the only part of this energy
  range that our eyes can detect. What we see is a
  rainbow of colors.
• RedOrangeYellowGreenBlueIndigoViolet
• ROY G BIV

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Frequency Ranges

• Wavelengths
• 104 101 1 10-2 10-5 10-6 10-8 10-10
  10-12
• Frequencies (cycles per sec)
• 3 x 106 3 x 1010 3 x 1014 3 x 1016
  3 x1018 3 x10 22

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Frequency Ranges of Visible Light

• Red light has a frequency of roughly
• 4.3 1014 Hz, and a wavelength of about 7.0
  107 m (700nm).
• Violet light, at the other end of the visible
  range, has nearly double the frequency7.5 1014
  Hzand (since the speed of light is the same in
  either case) just over half the wavelength
• 4.0 107 m (400nm).

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• The radiation to which our eyes are most
  sensitive has a wavelength near the middle of
  this range, at about
• 5.5 x 10-7m (550 nm), in the yellow-green region
  of the spectrum.

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• It is no coincidence that this wavelength falls
  within the range of wavelengths at which the Sun
  emits most of its electromagnetic energyour eyes
  have evolved to take greatest advantage of the
  available light.

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C f?

• The frequency (f) of a wave is the number of
  waves to cross a point in 1 second (units are
  Hertz cycles/sec or sec-1)
• ? is the wavelength- the distance from crest to
  crest on a wave

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• The product of wavelength and frequency always
  equals the speed of light.
• C f?
• Why does this make sense?
• NOTE
• c is a constant value 3.00 x 108 m/s

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PROBLEMS

• Calculate the wavelength of yellow light emitted
  from a sodium lamp if the frequency is
• 5.10 x 1014 Hz (5.10 x 1014 s-1)
• List the known info List the unknown
• c 3.00 x 1010 cm/s wavelength (?) ? cm
• Frequency (f) 5.10 x 1014 s-1
• C f? ? c
• f
• ? 3.00 x 1010 cm/s 5.88 x 10-5 cm
• 5.10 x 1014 s-1

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YOUR TURN

• 1- What is the wavelength of radiation with a
  frequency of 1.50 x 1013 s-1?
• 2- What frequency is radiation with a wavelength
  of 5.00 x 10-6 cm? In what region of the
  electromagnetic spectrum is this radiation?