EM Spectrum and Spectral Signatures

Types of spectra, electromagnetic spectrum, uses in identifying composition.

What are the regions of the EM spectrum in order?

Ultraviolet (UV), Visible, Near-Infrared (NIR), Shortwave Infrared (SWIR), Thermal Infrared (TIR), and Microwave.

What is a continuous spectrum?

A smooth, unbroken spread of all wavelengths (like a rainbow). It is produced by dense, hot objects (like stars or incandescent solids) and provides the “background light” for other spectral features.

What is an emission spectrum?

Bright lines at specific wavelengths caused when atoms or molecules release energy as photons. It identifies gases or excited elements (e.g., sodium emission lines in streetlights).

What is an absorption spectrum?

A continuous spectrum with dark lines or bands where specific wavelengths are absorbed by gases or molecules. These absorption features reveal the composition of atmospheres, surfaces, or materials.

Healthy vegetation — visible vs. NIR

Low reflectance in visible (chlorophyll absorbs red and blue, reflects some green). Sharp jump to high reflectance in NIR ("red edge") due to leaf mesophyll structure.

Stressed/dead vegetation — spectral signature

Reduced chlorophyll absorption → higher red reflectance. Reduced NIR reflectance as cell structure breaks down. Lower NDVI.

Water — spectral signature

High absorption across most wavelengths; especially strong in NIR and SWIR. Appears dark in NIR. Slight reflectance in blue/green for clear water; turbid water reflects more in green/red.

Snow/ice — spectral signature

Very high reflectance in visible (looks bright/white). Strong absorption in SWIR — this is why NDSI uses (Green − SWIR). Distinguishes snow from clouds, which reflect in SWIR.

Soil — spectral signature

Generally increases gradually from visible to NIR/SWIR. Moisture lowers reflectance. Iron content affects red. Distinguishable from vegetation by lack of red-edge jump.

Clouds — spectral signature

High reflectance across visible and NIR. Reflect in SWIR (unlike snow). Cold in thermal IR (especially high clouds). Used to separate clouds from snow.

Urban/built surfaces

Variable but generally moderate-to-high reflectance across visible and NIR; concrete and asphalt distinguishable in SWIR. Higher thermal emission (urban heat island).

Why is the sky blue? (Rayleigh scattering)

Air molecules scatter shorter wavelengths (blue) more strongly than longer (red).

Why are sunsets red?

At sunset, sunlight traverses more atmosphere. Blue light is scattered out of the line of sight; remaining red/orange light reaches the observer.

Mie vs. Rayleigh scattering

Rayleigh: particles much smaller than wavelength (air molecules); strong wavelength dependence. Mie: particles similar to wavelength (aerosols, water droplets); weaker wavelength dependence, scatters all wavelengths similarly (why clouds look white).

Nonselective scattering

particles much larger than the wavelength of light scatter all colors equally, causing fog and clouds to appear white or gray.

Why do plants appear green in visible light?

Because they absorb blue & red wavelengths for photosynthesis but reflect green.

How does vegetation appear in NIR, and why?

Vegetation is bright in NIR due to internal leaf cell structure reflecting strongly, which indicates plant health.

How does vegetation appear in SWIR, and why?

Vegetation appears dark in SWIR because leaf water absorbs strongly there. SWIR can be used to measure water stress.

How does water appear in NIR & SWIR?

Water appears very dark because it absorbs strongly in NIR and SWIR.

How does water reflect in visible light?

Reflects little in visible (looks blue because of scattering).

Which gases absorb in UV, IR, and SWIR/TIR?

• Ozone: absorbs UV.

• CO₂: absorbs in IR (esp. ~15 μm).

• H₂O vapor: absorbs in SWIR & TIR.