7.4.2—The approximate range of wavelengths in free space of the principal regions

The principal regions are radio, microwave, infrared, visible, ultraviolet, X-ray and gamma; wavelength decreases while frequency and photon energy increase across that order.

Use approximate ranges as scale guides, not as sharp universal borders. Match a region to an application only after checking the relevant wavelength or energy.

Microwaves have shorter wavelengths and higher frequencies than radio waves; X-rays are shorter and more energetic than ultraviolet.

The spectrum is continuous, and region labels are conventions rather than isolated gaps in wavelength.