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Copyright (c) John Lindsay, 2015

Readings

JR Jensen Chapter 2

Wave Properties of EM radiation

• Maxwell (1831-1879) conceptualized EMR as wave traveling through space at the speed of light, c.
  • c = 3x10⁸ m/s, enough to circle the Earth 7.5 times a second!
• The EM wave consists of two orthogonal fluctuating fields, one electric and one magnetic.

Wave Properties of EM radiation

Properties of EM radiation

• Wavelength (λ) – distance between maximums
• Frequency (ν) – number of wavelengths that pass per unit of time
• Long wave lengths = lower frequency and vice versa
• That is, ν is inversely proportional to λ such that:

\(\nu=\frac c\lambda\)

Properties of EM radiation

Common Units of Measurement

• Micrometre (not micrometer) μm: one millionth of a metre, 1 × 10^-6 m
• Nanometre (nm): one billionth of a metre, 1 × 10^-9 m or 1 × 10^-3 μm
• Angstrom (Å): 0.1 nanometre or 1 × 10^-10 m
• Also cm or metres for longer wavelengths

Common Units of Measurement

The Electromagnetic Spectrum

The Electromagnetic Spectrum

The Electromagnetic Spectrum

Portions of the Spectrum Important for Remote Sensing

• Ultraviolet: .10 μm — .40 μm
• Visible: .40 μm — .70 μm
  • Blue: .40 μm — .50 μm
  • Green: .50 μm — .60 μm
  • Red: .60 μm — .70 μm
• Infrared: (.7—1) μm — 1000 μm, i.e. 1 mm
  • Near or reflected: (.7—1) μm to 3 μm
  • Mid-infrared: 3 μm to (25–50) μm
  • Far or thermal: (25–50) μm to 1000 μm
• Microwave 0.3 cm to 30.0 cm
• Note: All ranges are approximate

The amount of energy associated with EMR is inversely related to its wavelength

(source: Jensen, 2007)

Interaction with matter

Reflectance

Reflectance is the ratio of energy reflected (bounced off) to the energy incident upon.

Reflectance

(source: NASA’s Observatorium, 1999)

Transmittance and Refractance

• Transmittance is the propagation of energy through a medium.
• Refractance occurs when EMR is transmitted through the interface between materials of different optical density.

Refraction is bending of light due to a change in speed

Absorptance

Why is the transmitted light from these filters coloured?

(source: http://www.aoe.com.au/filters.html)

Absorptance

• Absorbed energy is converted to some other form, e.g. heat.
• It is frequently re-radiated (emitted).

Atmospheric Scattering

• Results in a change in the path of a ray, but not the characteristics of light, i.e. the speed and wavelength.
• Rayleigh Scattering: diameter of matter is smaller (<0.1) than λ
• Mie Scattering: due to particles approximately equal to the λ (dust, pollen, water vapour)
• Non-selective Scattering: due to large particles (water droplets)

Atmospheric Scattering

(source: Jensen, 2007)

Atmospheric Scattering

(source: Unknown)

The blue sky results from the preferential scattering of the shorter blue wavelength of visible light...Rayleigh scattering!

Atmospheric Absorption

• Absorption occurs in the atmosphere due to interaction with water, carbon dioxide, ozone and nitrous oxide.
• This gives the atmosphere absorption bands where no EMR at a certain λ is available for remote sensing.

Atmospheric Windows

(source: Jensen, 2007)

Atmospheric Windows

(source: Jensen, 2007)