Definition and electromagnetic spectrum relationship

Summary

Infrared (IR) is electromagnetic radiation with wavelengths longer than visible red light but shorter than microwaves. There is no single universally accepted boundary, but it is commonly defined as roughly 780 nm (about 380 THz) to 1 mm (about 300 GHz). IR is invisible to the human eye because it begins just beyond the red edge of the visible spectrum, and it is often treated as part of the broader electromagnetic spectrum segment between visible light and microwaves. In the electromagnetic spectrum, IR sits directly below visible light and above microwaves, and it is frequently subdivided based on wavelength and typical physical behavior. A common practical split distinguishes longer-wavelength thermal IR (associated with heat emission from terrestrial sources) from shorter-wavelength near-IR (closer to the solar spectrum). Because the exact “start” of IR depends on detector and human-eye sensitivity, the visible-to-IR boundary is not sharply defined; the eye’s sensitivity drops rapidly beyond ~700 nm, while intense near-IR can still be perceived under certain conditions. The relationship between IR and the electromagnetic spectrum is also reflected in how IR radiation interacts with matter and how it is used: IR carries energy and momentum like other EM radiation, and it is emitted or absorbed by molecules during rotational-vibrational changes. This makes IR especially important for spectroscopy and for applications such as thermal imaging, night vision, and infrared astronomy, all of which rely on specific IR wavelength ranges within the spectrum.

Key Points

• 1

  Infrared is EM radiation with wavelengths longer than visible red and shorter than microwaves, commonly taken as ~780 nm to ~1 mm.

• 2

  IR is invisible to the human eye because it begins just beyond the visible red edge, and the boundary is not precisely fixed due to gradual changes in eye sensitivity and differing standards.

• 3

  IR is often divided into near-IR (shorter wavelengths, closer to solar spectrum) and thermal IR (longer wavelengths, strongly associated with heat emission).

• 4

  IR’s position in the EM spectrum determines its interactions with matter (e.g., molecular vibrational excitation) and enables uses like spectroscopy, thermal imaging, and infrared astronomy.