Why does ultraviolet light have to shine on white paper to be visible?

UV C LED flashlight jpg
Why does ultraviolet light have to shine on white paper to be visible?
 
The wavelength:
UV-C:222nm-250nm/250-260nm/260-270nm/270-280nm
UV-B:280-290nm/290-300nm/300-315nm
UV-A:320-330nm/330-340nm/340-350nm/350-360nm/360-370nm/370-380nm/380-390nm/390-400nm/400-410nm/410-420nm/420-430nm
 
Ultraviolet light is an electromagnetic radiation whose wavelength is shorter than visible light and cannot be directly perceived by the human eye. When ultraviolet rays are irradiated on the white paper, the white paper will absorb the ultraviolet rays and fluoresce. Fluorescence means that after a substance absorbs energy, it emits light with a wavelength longer than the absorbed energy. On white paper, fluorescence converts ultraviolet light into visible light, allowing the human eye to see the presence of light. Therefore, we can only see the effect of light when the ultraviolet light shines on the white paper.
 

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