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Copyright (c) John Lindsay, 2015


The Earth From Space

Electromagnetic Radiation
and the Spectrum

John Lindsay
Fall 2015


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.

Jensen's EM model (source: Jensen, 2007)

Wave Properties of EM radiation

GIF of EM radiation (source:

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

Jensen wavelength and frequency
(source: Jensen, 2007)

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

Scale example

The Electromagnetic Spectrum

The EM spectrum

(source: "EM Spectrum Properties edit" by Inductiveload, information by NASA)

The Electromagnetic Spectrum

The EM spectrum

(source: Christopherson and Byrne, 2008)
The EM spectrum split

The Electromagnetic Spectrum

The EM spectrum split

(source: Jensen, 2007)

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

The EM spectrum and energy
(source: Jensen, 2007)

Interaction with matter

EMR interaction with matter


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

Types of reflectance


Spectral signature
(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

Common refraction instances


Why is the transmitted light from these filters coloured?



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

Thermal IR image

Interaction with the atmosphere

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

Scattering types Scattering by wavelength
(source: Jensen, 2007)

Atmospheric Scattering

Scattering effects on image
(source: Unknown)

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

Color of our sun

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

Atmospheric windows
(source: Jensen, 2007)

Atmospheric Windows

Atmospheric windows and solar radiation
(source: Jensen, 2007)