Ultraviolet Light: Now is the story of the ultraviolet light.  Though we usually classify the infrared (IR), ultraviolet (UV), and visible lights on the basis of their wavelength, the light can be classified by the corresponding frequency.  As the frequency of a light equals “the light velocity (c=299 792 458 m/s) divided  by the wavelength of the light”, the frequency of the UV light whose wavelength is shorter than that of a visible light is higher than that of the visible light.  As the energy carried by a photon, a basic element of the electromagnetic radiation, is proportional to the frequency of it, it may be better sometimes to classify the kind of the light by the frequency.  It should be kept in mind that the light is an elementary particle (a photon) as well as an electromagnetic wave.   The ranges of the wavelength and the frequency of the IR, the visible, and the UV lights are summarized in the following.

    The infrared light：         1 mm - 700 nm ,      0.3 THz -   400 THz

    The visible light：        700 nm - 400 nm ,    400 THz -    750 THz

    The ultraviolet light： 400 nm -    10 nm ,    750 THz -  3000 THz

where 1THz=1 000 000 000 000Hz and 1Hz means one oscillation per a second.  It should be noted that the boundary frequency (the boundary wavelength) between these lights are not necessarily defined strictly.  By expressing the light by its frequency it is clearly understandable that the UV light has higher energy in comparison with the IR and the visible light.  The energy of the UV light is from about 3 eV (electron volt) to about 100 eV, which is appropriate for inducing reactions among atoms and molecules.  For this reason various kinds of chemical reactions are caused by the energy of the UV light, and living bodies are affected by the UV light whether it's beneficial or damaging.  Moreover, the UV light is subdivided into several different subclasses as UV-A (long wavelength ultraviolet, 400 - 315 nm, 3.10 - 3.94 eV), UV-B (medium wavelength ultraviolet, 315 - 280 nm, 3.94 - 4.43 eV), UV-C (short wavelength ultraviolet, 280 - 100 nm, 4.43 - 12.4 eV) and V-UV (vacuum ultraviolet, far ultraviolet, 100 - 10 nm, 12.4 - 124 eV).

Human eyes and the ultraviolet light: Human eyes cannot usually sense the UV light because the horny coat and the crystal lens of an eye absorb the UV light.  A cataract sufferer who had an operation of intraocular lens implantation is said to be able to sense the long wavelength UV light with the wavelength as low as 300 nm (Eugene Hecht, Optics 4th Edition, Addison Wesley 2002).  Actually, I had the cataract surgery and intraocular lenses are implanted, but up to now I have not been recognizant to sensing the UV light through my eyes.  As the wavelength region of 300 nm - 400 nm corresponds to the region which the sensor of a digital camera senses, one who had a cataract operation may see the similar world of a ultraviolet photograph.  A reason why we, cataract sufferers, are not aware of the ultraviolet light despite of this fact is that the predominant visible light covers over the existence of the world of the UV light.  However, it is often said that things around look blueish after cataract surgery and UV blocking glasses are commercially available for this reason.  Therefore, I may be just simply dull.  I have wandered from the subject point.