Spectroscopy and the electromagnetic spectrum

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.

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.