Electromagnetic%20Radiation

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Chapter 6 Electromagnetic Radiation
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Short wavelength --gt high frequency high
energy

• Long wavelength --gt
• small frequency
• low energy

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The electromagnetic spectrum.
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Which has the longest wavelength?

1. Infrared
2. Ultraviolet
3. X-rays
4. Radio waves

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• Rank the following in order of increasing
  frequency
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• microwaves
• radiowaves
• X-rays
• blue light
• red light
• UV light
• IR light

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• Waves have a frequency
• Use the Greek letter nu, ?, for frequency, and
  units are cycles per sec
• All radiation ? ? c
• c velocity of light 3.00 x 108 m/sec
• Long wavelength ?small frequency
• Short wavelength ?high frequency

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The Wave Nature of Light

• The product of the frequency, n (waves/sec) and
  the wavelength, l (m/wave) would give the speed
  of the wave in m/s.

• So, given the frequency of light, its wavelength
  can be calculated, or vice versa.

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The Wave Nature of Light

• What is the wavelength of yellow light with a
  frequency of 5.09 x 1014 s-1? (Note s-1,
  commonly referred to as Hertz (Hz) is defined as
  cycles or waves per second.)

• If c nl, then rearranging, we obtain l c/n

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The Wave Nature of Light

• What is the frequency of violet light with a
  wavelength of 408 nm?

• If c nl, then rearranging, we obtain n c/l.

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• What is the wavelength of WONY?
• What is the wavelength of cell phone radiation?
  Frequency 850 MHz
• What is the wavelength of a microwave oven?
  Frequency 2.45 GHz

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Quantization of Energy
Light acts as if it consists of particles called
PHOTONS, with discrete energy.

• Energy of radiation is proportional to frequency

E h ?
h Plancks constant 6.6262 x 10-34 Js
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E h ?
Relationships
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Short wavelength light has

1. High frequency and low energy
2. High frequency and high energy
3. Low frequency and low energy
4. Low frequency and high energy

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• Rank the following in order of increasing photon
  energy
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• microwaves
• radiowaves
• X-rays
• blue light
• red light
• UV light
• IR light

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Energy of Radiation

• What is the frequency of UV light with a
  wavelength of 230 nm?
• What is the energy of 1 photon of UV light with
  wavelength 230 nm?

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What is the energy of a photon of 525 nm light?

1. 3.79 x 10-19 J
2. 4.83 x 10-22 J
3. 3.67 x 1020 J
4. 8.43 x 1023 J

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Radio Wave Energy

• What is the energy of a photon corresponding to
  radio waves of frequency 1.255 x 10 6 s-1?

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• What is the energy of a mole of 230 nm photons?
• Can this light break C-C bonds with an energy of
  346 kJ/mol?

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• Does 1200 nm light have enough energy to break
  C-C bonds?

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Where does light come from?

• Excited solids emit a continuous spectrum of
  light
• Excited gas-phase atoms emit only specific
  wavelengths of light (lines)

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Light emitted by solids
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Light emitted by hydrogen gas
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The Bohr Model of Hydrogen Atom

• Light absorbed or emitted is from electrons
  moving between energy levels
• Only certain energies are observed
• Therefore, only certain energy levels exist
• This is the Quanitization of energy levels

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Emission spectra of gaseous atoms

• Excited atoms emit light of only certain
  wavelengths
• The wavelengths of emitted light depend on the
  element.

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Line spectra of atoms
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Energy Adsorption/Emission
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• For H, the energy levels correspond to

Constant 2.18 x 10-18 J
Energy level diagram
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Each line corresponds to a transition

• Example n3 ? n 2

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Explanation of line spectra
Balmer series