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## GEOG*2420

### 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.

(source: Jensen, 2007)

### Wave Properties of EM radiation

(source: http://www.molphys.leidenuniv.nl/monos/smo/index.html?basics/light_anim.htm)

• 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:

### $$\nu=\frac c\lambda$$

(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

### The Electromagnetic Spectrum

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

### The Electromagnetic Spectrum

(source: Christopherson and Byrne, 2008)

### The Electromagnetic Spectrum

(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

(source: Jensen, 2007)

### 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)