Why Does CFL Lighting Cause a Violet Line in Laundry Detergent?

[JFS321]

All, I'm looking for a little hypothesis help here. I know that most detergents these days contain optical brighteners--they absorb UV and re-emit visible light the blue violet region. I noticed, though, that looking at my laundry detergent under a CFL bulb produces this rather neat violet line at the top of the detergent.

So, I'm wondering why this is the case. Holding the detergent in front of a sunny window does not produce the violet line, but turning on a CFL above the sunny window does. I haven't been able to see if the line is produced by LEDs or incandescent bulbs yet. My guess is that CFLs could not possibly be emitting enough UV to cause this to fluoresce at the top, but CFLs must emit light at wavelengths that the optical brighteners are absorbing and re-emitting...so there must be some overlap between the UV and visible range that the optical brighteners can absorb.

Thoughts?

 

[Ygggdrasil]

Glass absorbs UV light fairly well, so that would explain why you don't see the fluorescence with sunlight filtered through the window. UV emission from the CFL could be responsible:

Fluorescent lamps, large and compact, work by using electricity to excite mercury vapor inside the bulb. The excited vapor then emits invisible ultraviolet light that is absorbed by the bulb's phosphor coating. In turn, the coating re-emits the energy as visible light.

But researchers found UV light leaks more from CFLs compared to standard fluorescent tubes because the small diameter of the glass coupled with its twists and turns creates more spaces where the phosphor coating chips away, letting more UV light escape.

http://www.scientificamerican.com/article/can-compact-fluorescent-lightbulbs-damage-skin/

 

[Kevin McHugh]

The violet line could also be due to refraction at the liquid/air interface.