Can some someone plz help

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Ultra violet radiation

Explanation:  Sun is the major source of these radiations. Ultraviolet  Radiation has the range of 10nm-400nm and are responsible to make the things or spots glow which cannot be seen with the naked eye. These pens are usually fluorescent but are transparent which cn only be seen under the radiation.

This the reason why ultra violet radiations are used for security purposes and to prevent any illegal cause.