Near a UV lamp, a white T-shirt can give off a striking blue-white glow. The main cause is usually not the cotton fibre itself, but fluorescent brightening agents applied during textile finishing or left behind by laundry detergent. These molecules absorb near-ultraviolet light, which our eyes cannot see. Their electrons briefly enter a higher-energy state; after some energy is lost through molecular vibration and other processes, the remainder is released as longer-wavelength blue visible light. This is fluorescence, which normally stops quickly when illumination ends, unlike phosphorescence that can persist.
The blue emission also offsets the slight yellow cast of ageing white fabric. When daylight supplies some ultraviolet radiation, the brightener adds a little blue light, making the cloth look whiter and brighter. A UV lamp isolates and strengthens that mechanism, so the blue glow becomes conspicuous. Untreated natural fibres, and clothes with different washing histories, may therefore glow with very different intensities.
The effect shows why looking whiter is not the same as being chemically cleaner. A brightener changes the spectrum reaching the eye; it does not replace the surfactants that remove oily dirt. UV sources should not be stared at or used for prolonged skin exposure. Any observation should use a compliant low-power device at the distance specified by its instructions.
https://doi.org/10.2307/4451992
https://www.gia.edu/gems-gemology/summer-2020-optical-whitening-brightening-of-pearls
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