How to detect fake currency notes by UV ray?
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Wavelength of UV ray is from A = 10nm to 380 nm. But UV ray with wavelength is used as optical sensor in true false currency in note detection machine. Special chemical substance named optical sensor phosphor is used as security device in currency note. Electron of phosphor molecule being excited by low wavelength or high frequency UV ray is transferred to higher energy level. At next moment the excited electron of phosphor molecule returns back to stable state from excited state and the absorbed energy is radiated as visible light of definite frequency. As the radiation is with color so it is called fluorescence. Based on this principle, UV ray is used to detect fake note/passport.