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For convenience in talking about electromagnetic radiation, we classify photons of different energies into different spectral regions. The photons in all of these regions have the same electromagnetic nature, 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). These photons are 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 our eyes can sense the 400-nm photons directly. A 350-nm photon is in the ultraviolet portion of the electromagnetic spectrum. Some of the boundaries between spectral regions are not well-defined as between ultraviolet and visible radiation.
The visible region of the electromagnetic spectrum consists of photons with wavelengths from approximately 400 to 700 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 700 nm can be seen if the light source is intense.
The following table lists the names of different spectral regions, the range of frequencies and wavelengths in those regions, and the type of transition that can occur when a photon in these spectral ranges interacts with matter.
|Type of Radiation||Frequency Range (Hz)||Wavelength Range||Type of Transition|
|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
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