Electromagnetic Radiation
Electromagnetic Radiation
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Is light a wave or a particle?
Yes
It’s 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)
Light Waves
• Light consists of
oscillating electric and
magnetic fields
• Changing electric field
creates changing
magnetic field …..
• Mostly we visualize the
electric field – it’s
simpler
Waves and Energy
• Frequency * Wavelength = c (speed of light)
• Speed of light in materials < 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 = h*frequency
– h = Planck’s Constant
• High Frequency = Short Wavelength = High
Energy
Fig. 18-2, p.430
Light and Materials
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Transmission
Absorption
Reflectance
Scattering
– Related to roughness and particle size
• Emission
– Mostly infrared
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
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
Black Body Radiation
What’s The Source of the Light?
Color = Temperature
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
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
Atoms and Radiation
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
The Solar Spectrum
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