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Lecture 2: Basic principles of electromagnetic magnetic radiation (EMR). Prepared by Rick Lathrop 9/99 Updated 9/04. Basic interactions between EMR and the earth surface. Reflection: specular reflection or scattering Absorption Transmission. q 1 q 2. q 1 = q 2.
Prepared by Rick Lathrop 9/99
specular reflection or scattering
q1 = q2
EMR re-emitted as thermal energy
Shorter ls refracted more
Adapted from Lillesand & Kiefer Remote Sensing and Image Interpretation
l = wavelength
l = 10 mm
n = 1013 Hz
l = 1.0 mm
n = 1014 Hz
l = 0.1 mm
n = 1015 Hz
Example: n = 600 Mhz l = ?
n = c / l or l = c /n
l = 3 x 108 m/sec / 600 x 106 hz =
l = 3 x 108 m/sec / 6 x 108 hz = 0.5 m
What EMR region is this wavelength? microwave
Comparative Sizes: from subatomic to human scales
Human & larger
adapted from NY Times graphic 4/8/2003
Incidentally if the peak of sunlight energy is in the shorter visible wavelengths, why did Herschel find the infrared to be hotter. Due to the nonlinear nature of refraction, his prism concentrated the infrared light, while dispersing the shorter wavelength visible colors.
Why do UV and not NIR rays cause sunburn?
UV has approximately 3x the amount of energy per quanta
Which emits more energy – the Sun or Earth?
= 7.35 x 107 W m-2
- (5.6697 x 10-8 W m-2 K-4)(3000K)4 = 4.59 x 102 W m-2
Relationship between temperature and EMRObjects emit energy over a range of wavelengths. As the temperature of the object increases, its radiant flux increases. The wavelength of maximum flux depends on the temperature of the object.
Blackbody at temperature T1
T1 > T2
Blackbody at temperature T2
Why is the outside of a candle’s flame red, while the inner flame is blue?
where A = 2898 mm K
Why do humans see in the ‘visible’ and not the NIR?
For more info on color vision go to:
Why is the sky blue and clouds white?
Graphic from http://earthobservatory.nasa.gov/Library/RemoteSensingAtmosphere/
0.4 0.5 0.6 0.7 0.8 1.1 umAtmospheric interference with EMR
Signal decreased by absorption
Signal increased by scattering
Adapted from Jensen, 1996, Introductory Digital Image Processing
Clouds scatter all ls of visible light, appear white
Air molecules scatter short l blue light, longer ls transmitted
Why are plants green?
Chlorophyll pigment is contained in minute structures called plastids that are found in the leave’s parenchyma cells. Chlorophyll differentially absorbs red and blue wavelengths of light, there is less absorption in the green and nearly no absorption in the near IR.
Graphic from: http://iusd.k12.ca.us/uhs/cs2/leaf_cross-section.htm
Graphic from: http://www.olympusmicro.com/primer/lightandcolor/refraction.html
How plant leaves reflect lightAs light moves from a hydrated cell to an intercellular space it gets refracted, sometimes multiple times. Eventually, some light may be scattered back out through the upper leaf surface and some transmitted down through the leaf.
NIR light (which is not absorbed) is scattered within leaf: some reflected back, some transmitted through
Blue & red light strongly absorbed by chlorophyll. Green light is not as strongly absorbed
Cross-section of leaf
Spectral reflectance characteristics are both spatially and temporally variable. For example, each leaf (even within the same species) is different and can change. Thus you should think of a spectral signature as more as a spectral “envelope”.
Why do plants turn yellow as they senesce?
A leaf’s chlorophyll (1) begins to break down as the leaf senesces (as in the autumn). Accessory plant pigments (such as carotenoids and anthocyanins) are also found in the leaf cells but are generally masked by chlorophyll. Without chlorophyll, these pigments dominate. Carotenoids absorb blue to blue green wavelengths and thus appear yellow to orange (2). Anthocyanins absorb blue to green wavelengths and thus appear magenta (purple) to red (3) .
Graphic from: http://www.fs.fed.us/conf/fall/leafchng_nf.htm
lm = wavelength of max radiant exittance
A = 2898 mm K T = temperature K