How is an electromagnetic wave used in the detection of invisible writing ?

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Solution

Dear Student,
Some inks glow faintly (fluoresce) when under an ultraviolet lamp. This is a property of many substances, particularly organic substances and body fluids.
Other inks work in a near opposite way by absorbing ultraviolet light but without fluorescing. When these are used on fluorescent paper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint.
Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.

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