For convenience in talking about radiation, we divide electromagnetic radiation into different spectral regions. The radiation in all of these regions are still electromagnetic waves, but because of their very different energies they interact with matter very differently. For example, the human eye can only detect radiation that is in the visible region of the spectrum (hence the name), which is both transmitted by the lens of the human eye and absorbed by the photoreceptors in the retina. There is no fundamental difference in the nature of electromagnetic radiation of 350 nm versus 400 nm, other than we can see the 400 nm photons directly. Some of the boundaries between regions are not well-defined.
The visible region of the electromagnetic radiation is approximately 400 to 750 nm. The short wavelength cutoff is due to absorption by the lens of the eye and the long wavelength cutoff is due to the decrease in sensitivity of the photoreceptors in the retina for longer wavelengths. Light at wavelengths longer than 750 nm can be seen if the light source is intense.
| Type of Radiation | Frequency Range (Hz) | Wavelength Range | Type of Transition |
|---|---|---|---|
| gamma-rays | 1020-1024 | <1 pm | nuclear |
| X-rays | 1017-1020 | 1 nm-1 pm | inner electron |
| ultraviolet | 1015-1017 | 400 nm-1 nm | outer electron |
| visible | 4-7.5x1014 | 750 nm-400 nm | outer electron |
| near-infrared | 1x1014-4x1014 | 2.5 µm-750 nm | outer electron molecular vibrations |
| infrared | 1013-1014 | 25 µm-2.5 µm | molecular vibrations |
| microwaves | 3x1011-1013 | 1 mm-25 µm | molecular rotations, electron spin flips* |
| radio waves | <3x1011 | >1 mm | nuclear spin flips* |
*for energy levels split by a magnetic field