Chapter 2 Flashcards
ω λ wavelength f/v frequency μm micrometer τ transmissivity ξ absorptivity θ theta
EMR defined
- Form of energy transport in free space
* wave travel through space at speed of light
Maxwell equations
Esubx = Eomega cos (wt-kz)
Eomega = max electrical energy ω= angular freq (2 piv)
Electric and magnetic fields are ________ to each other
Orthagonal (perpendicular?)
Radiant energy (Q of a photon) is proportional to frequency
Q=hv
Q=radiant energy (J)
v=frequency
h=Plank’s constant (6.626 X 10^-34 Js
substituteQ=hc/wavelength
h is small number, so approx Q~ 1/wavelength
Radiation from the Sun is ________________ (re polarization)
un-polarized
Man-made sources (laser, radar) have ___________ radiation (re polarization)
polarized
Electromagnetic spectrum for remote sensing
UV, visible, near-ir, mid-ir, thermal, microwave bands (Ka, Ku, X, C, S, L, P)–LOOK AT GRAPH FROM LECTURE
shorthand ranges for RGB spectra
blue .4-.5 μm
green .5-.6 μm
red .6-.7 μm
Polarization
The orientation of the electric field
Vertical polarization: electric vector is PERPENDICULAR to the plane of incidence
Horizontal polarization: electric vector is PARALLEL to the plane of incidence
Sun=unpolarized
Man-made sources (laser, radar) have polarized radiation
Infrared
near and mid=reflective =short wave
far=emissive radiative thermal
Microwaves
letters come from military, don’t want enemy to know signal
The bulk of sun’s radient energy distribution is
visible (43.5%) and near infrared (36.8%). Also significant amts of near UV (5.32%) and Mid IR (12%)
Blackbody concept
Object that absorbs and emits 100% of radiation
Does not exist in nature
Emissivity would equal 1
Assumptions:
- Isotropic
- homegeneous
- unpolarized
2 objects with same temp would emit same E
Blackbody would emit more E than a comparable gray body (which has Emissivity is < 1)
MOST IMPORT. CONCEPT
Graybody
Object that reflects part of the incident radiant
Emissivity is < 1
M=emissivitysigma constantTemp4 (double check this)
Emissivity
the relative ability of a surface to emit radiation
- describes ACTUAL absorption and emission properties of real objects (gray bodies)
- Is wavelength dependent (usually use avg)
- Is equal to (graybody emittance)/(blackbody emittance)–of same temp
- Use it to calculate an object’s radiant temp, or brightness temp
The temp at which a blackbody would have to be to emit the same energy as emitted a graybody at some physical temp
T(rad) = e^(1/4)T(kin) e= [(T(rad) / T(kin)] ^4
kin=kinematic physical
Selective radiator
emits certain types of EMR (better)
Two objects can have the same ___________ temp but different _________ temperatures…Why?
kinematic, radiant
Because they have different emissitivities
Understand why mirror has no emissivity
gah
Planck’s law (Spectral radiance)
‘All bodies who temp are above absolute zero K (-273.2), emit radiation”
Heat transformed into radiant energy
(need formula)
L = amount of E per unit serface per unit time, per solid angle emitted at the wavelength λ
Maximum radiation of sun at what wavelength?
6000K (Kelvin) at ~.5 micrometer
Stefan-Bolzmann law (TIR)
The total emitted energy over the whole spectrum is proportional to the physical temperature.
The amount of energy emitted by an object such as the Sun or Earth is a function of its temperature. ^temp=^emitted energy
Wien’s law
That wavelength of peak emittance (max wavelength) is inversely proportional to an object’s kinematic temperature (derivative of Planck’s law)
λmax = a/T
a=2898 μm K
e.g. hotter the object, the shorter wavelength of maximum intensity
False color
Different channels represented with color not true to visible light. May more clearly delineate areas
Solid angle
imagine as angle with three dimensions (cone)
Look over definition of radiation quantities
tba (know what is what)
EMR interaction with matter
Radiative properties of a natural surface
• Radiation incident upon a surface must either be transmitted (t) through it, reflected (a) from the surface or be absorbed (ξ).
Transmissivity (τsubλ) + Reflectivity (αsubλ) + Absorptivity (ξsubλ) = 1
- For solar radiation, a is referred to as the surface albedo
- If we consider only part of the EM spectrum, a is referred to as the spectral albedo
albedo
- For solar radiation, Absorptivity (a) is referred to as the surface albedo
- If we consider only part of the EM spectrum, a is referred to as the spectral albedo
transmission
Transmission
– incident radiation passes through the material without attenuation
– change in the direction of radiation is given by the index of refraction of the material
– Index of refraction (n) is the ratio of the speed of light in a vacuum relative to the speed of light through the material
n = c/c subn
Snell’s Law describes relationship between incidence and refraction angles:
Therefore
n1 sinq1 = n2 sinq2 n1/n2 = sinθ2/sinθ1
Snell’s Law
Snell’s Law describes relationship between incidence and refraction angles:
n1 sinθ1 = n2 sinθ2
Therefore
n1/n2 = sinθ2/sinθ1
Reflection
Four kinds kinds (all can happen at same time):
• Reflection (Specular Reflection)
– Surface is smooth relative to wavelengths (smooth relative to the wavelength)
– Mirror-like surfaces are called specular reflectors
• Scattering (Diffuse Reflection)
– Surface is rough relative to wavelengths
– EMR velocity and wavelength are not affected
- Absorption
- Attenuation
Absorption
– Substance is opaque to incident radiation
– Portion of EMR is converted to heat energy (re-radiated)
Attenuation
Weakening of EMR as it passes through a medium. Sometimes called extinction.
– Combination of absorption and scattering
Plants absorb/reflect what
absorb blue and red, reflect green and infrared
Water absorbs/reflects what?
reflects blue, green, red, IR, in progressively less amounts, until absorption at approx 800 nm
Atmospheric Effects
• EMR is attenuated by its passage through atmosphere
• Attenuation= scattering + absorption
- Scattering is the redirection of radiation by reflection and refraction
- Attenuation is wavelength dependent
- Decrease with increase in wavelength
Three types of scattering in the atmosphere
Rayliegh, Mie, and non-sective scattering
The type of scattering is a function of
1) wavelength
2) size of the gas molecule, dust particle, and/or water vapor droplet encountered
Rayleigh scattering
(molecular scattering)
• Scattering by molecules and particles whose diameters are «_space;wavelength
• Primarily due to oxygen (O2) and nitrogen (N2) molecules
• Scattering intensity is proportional to λ-4
• Responsible for blue sky (blue is shorter than other)
Mie scattering
• Particles that have a mean diameter of 0.1 to ten times the incident wavelength
– Examples: water vapor, smoke particles, fine dust
– Scattering intensity is proportional to anywhere from λ-4 to
λsup0 (depending on particle size)
• Clear (all colors) atmosphere has both Rayleigh and Mie scattering. Their combined influence is between λ-0.7 and λ-2
Why we see red if there’s a fire or red sky–Mie scattering is favoring the longer wavelengths.
Red sky = angle in atmosphere means Rayleigh scatters more, so blue reflected into space.
Non-selective scattering
- All visible spectra scattered (then will have white/gray)
• Aerosol particles much larger than wavelength (>10x)
• Examples: water droplets, ice crystals, volcanic ash, smog
• Independent of wavelength λ0
Why does Rayleigh scattering make the sky blue?
Blue wavelength is shorter, so the blue light is scattered 4x more than red. Thus, the intensity of the blue is larger than the red (divided by smaller number).
Why does Mie scattering make the sky red?
Mie scattering is favoring the longer wavelengths.
What is difference between white and gray
more or less reflection/absorption. White scatters all of visible light. Gray reflects some of all visible light.
Atmospheric Absorption
• In the atmosphere, EM radiation is absorbed by: – H2O Water vapor, Water droplets – CO2 Carbon dioxide – O2 Oxygen (not very effective) – O3 Ozone
• This absorption is the transfer of electromagnetic energy to the molecules with which the EMR comes in contact with
Vibrational Processes (how absorption occurs)
- Small displacements of atoms from their equilibrium position, resulting from absorption of energy
- N atoms –> 3N possible vibrational modes
- Figures show three modes for the water molecule (symmetric OH stretch, assymetric OH stretch, HOH bend)
Vibrational Processes of water (three classical frequencies)
The water molecule has three classical frequencies (v1, v2, v3) which correspond to three wavelengths:
- 3.106 μm (symmetric OH stretch)
- 6.08 μm (HOH bend)
- 2.903 μm (asymmetric OH stretch)
An example of combination: v=v3 +v2
1/λ = 1/λ3 + 1/λ2 = 0.5089 –> = 1.965 μm
• Major spectral regions pertinent to remote sensing (see graph)
used in lab
The nature and amount of atmospheric absorption depends on (three):
- Absorption spectra of the atmospheric gases
- Clouds
- Aerosols
*** Atmospheric Windows
Regions in the EM spectrum where energy can be fully transmitted (pass through the atmosphere because little absorption)
- 0.3-0.7 mm –UV and visible light
- 3-5 mm – emitted thermal energy from Earth
- 8-11/14 mm –emitted thermal energy from Earth
- 1 mm-1 m – radar and microwave energy
Reviewed from last week
transmissivity + reflectivity + absorptivity = 1
albedo
Snell’s law
albedo–surface vs spectral
tba
which in EMR are reflected?
visible, near IR
Which are emitted?
Thermal IR and microwave
