Electromagnetic Radiation (EMR), Atmospheric Windows & Spectral Signatures

Explanation

Every material on Earth exhibits a unique spectral reflectance curve governed by its molecular composition. Chlorophyll absorbs blue and red wavelengths for photosynthesis, reflecting green light. In the Near-Infrared (NIR) band, the internal cellular structure of healthy leaves reflects up to 50% of incoming radiation to prevent thermal overheating. As vegetation undergoes drought stress or disease, the spongy mesophyll collapses, causing an immediate drop in NIR reflectance long before visible discoloration appears. Water absorbs nearly all infrared radiation, appearing black in NIR imagery.

Key points

• Blackbody Radiation Laws: Planck's Law (radiation distribution), Stefan-Boltzmann Law (E = sigma * T^4), Wien's Displacement Law (lambda_max = 2898 / T um).
• Sun's peak emission occurs in the visible spectrum (~0.5 um, 6000 K); Earth's thermal emission peaks in the thermal infrared (~9.7 um, 300 K).
• Atmospheric Windows: Wavelength bands where the atmosphere is transparent (minimal absorption by water vapor, ozone, carbon dioxide).
• Atmospheric Scattering: Rayleigh (particles smaller than wavelength - causes blue sky), Mie (smoke/dust of equal size), Non-selective (water droplets larger than wavelength - causes white clouds).
• Spectral Signatures: Distinctive reflectance curves across wavelengths: Healthy Green Vegetation (low in blue/red due to chlorophyll absorption, high peak in Green, massive plateau in Near-Infrared ~0.7-1.1 um due to spongy mesophyll cellular structure).

Example

Code examples

/*
Spectral Reflectance Curves Overview:
Wavelength Band | Vegetation (Healthy) | Clear Water         | Bare Soil
Blue (0.45 um)  | Low (~5%)            | Moderate (~10%)     | Low (~10%)
Green (0.55 um) | Moderate Peak (~15%) | Peak (~15%)         | Moderate (~15%)
Red (0.65 um)   | Low Absorption (~5%) | Low (~3%)           | Increasing (~25%)
NIR (0.85 um)   | Massive Peak (~50%)  | Complete Absorb (0%)| High (~35%)
SWIR (1.6 um)   | Water absorp dips    | Zero (0%)           | High Peak (~45%)
*/

Exam questions

• [10 Marks] Explain the physical properties of Electromagnetic Radiation (EMR). State Wien's Displacement Law and Stefan-Boltzmann Law.
• [10 Marks] Draw and thoroughly explain the Spectral Reflectance Curves for healthy green vegetation, bare dry soil, and clear deep water across visible, NIR, and SWIR bands.