Electromagnetic Radiation

1
Electromagnetic Radiation
2
Electromagnetic Radiation

• Is light a wave or a particle?
• Yes
• Its both, and neither
• At atomic scales, we have no exact analogs for
  phenomena
• For some purposes light is best treated as a
  wave, for others as particles (photons)

3
Light Waves

• Light consists of oscillating electric and
  magnetic fields
• Changing electric field creates changing magnetic
  field ..
• Mostly we visualize the electric field its
  simpler

4
Waves and Energy

• Frequency Wavelength c (speed of light)
• Speed of light in materials lt c
• In air, the difference is tiny, but critical in
  surveying
• When light changes speed, it changes direction
• Refraction of light causes distortion
• Energy of a photon hfrequency
• h Plancks Constant
• High Frequency Short Wavelength High Energy

5
Fig. 18-2, p.430
6
Light and Materials

• Transmission
• Absorption
• Reflectance
• Scattering
• Related to roughness and particle size
• Emission
• Mostly infrared

7
Sources of Radiation

• Black body
• Hot objects glow visibly
• Warm objects emit infrared and radio
• Useful for remote temperature sensing
• Thermal emission also useful for characterizing
  surfaces

8
Black-Body Radiation

• Objects Emit Radiation Because They Are Hot
• Why Black? Because None of the Radiation is
  Reflected from Some Other Source
• The Sun Emits Black-Body Radiation, Mars Does Not
• Close Example of pure Black-Body radiation
  Peephole in a pottery kiln

9
Black Body Radiation
10
Whats The Source of the Light?
11
Color Temperature
12
Why Black-Body Radiation is so Important

• Color is directly related to temperature
• Temperature is the only determinant of color
• Energy per unit area is the same if temperature
  is the same
• If two stars have the same color and distance,
  difference in brightness is due to difference in
  size
• Dwarf and giant stars are literally dwarfs or
  giants

13
Sources of Radiation

• Molecular vibrations
• Mostly infrared
• Create absorption bands in atmosphere
• Active sensing with tuned lasers
• Atomic excitation
• Largely visible light
• Many mechanisms at work
• How we see

14
Atoms and Radiation
15
Spectroscopy

• Different atoms absorb or emit specific
  wavelengths of light
• When light spread into a spectrum, the absorbed
  wavelengths show up as dark (missing) bands
• These spectral lines are indicators of
• Chemical composition
• Physical conditions

16
The Solar Spectrum
17
Remote Sensing and EM

• Microwaves Used in radar imaging
• Infrared
• Absorption by molecules
• Emission by warm materials
• Reflectance
• Visible Light
• Ultraviolet
• Absorbed by ozone in atmosphere
• Most rocks absorb UV strongly
• Mostly useful in astronomy for high energy
  phenomena