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Types of photons

Photons are classified into broad categories delineated by their wavelengths. This is equivalent to classifying them by how much energy they have, as I explained in the previous section.

Radio waves are the lowest energy photons. These photons have wavelengths longer than approximately 1 meter.
Microwave photons have wavelengths ranging from about 1 meter to 1 millimeter. (Some people consider microwave photons to be a type of radio wave--this is semantics and is unimportant.)
Infrared radiation consists of photons with wavelengths between about 1 millimeter and 0.8 microns (0.0008 mm). Some researchers divide infrared radiation into three bands. Ordered from short to long wavelengths these bands are near (or reflected) infrared, thermal infrared, and far infrared.
Visible light wavelengths are about 0.4-0.8 microns.
Ultraviolet radiation wavelengths range from about 0.38 microns to 1 nanometer.
X-ray wavelengths are about 1 nm to 0.001 nm.
Gamma rays are any photon that has a wavelength smaller than X-rays.

As you probably noticed, scientists use different units of measurement for the wavelengths of different types of photons. This is because photons vary over such a wide range of wavelengths that units appropriate for one kind of photon wavelength are annoying for another kind of photon. (Consider, for example, the clumsiness in specifying the wavelength of a radio wave as 100,000,000,000 nm, or an ultraviolet photon as 0.001 microns. These would be better written as 100 m and 1 nm, respectively.)

Conversions among the commonly used units (especially microns and nanometers) are:

1 meter=10⁶ microns,
1 mm=10³ microns=10⁶ nanometers,
1 mm=10⁷ angstroms.