Fluorescent colours and the atmosphere

[tishk]

Hi guys. I'm an anthropologist really, and won't pretend I know the finer points of physics. still, this thing bothers me, and hopefully someone on this forum thinks it's an easy question.

I noticed a while back that local crafts in highland cultures - Himalayan and Andean alike! - made use of Day/Glo, neon, fluorescent colours. I think I have figured out that there is more UV light in high altitudes due to thinner atmosphere, although I don't seem to have very good sources for it. If anyone has read anything that could be used as a respectable reference, make my day.

The question that perplexes me, tho, has to do with how, while the general idea for observing fluorescence without a blacklight seems to be to conduct the experiment at noon and in direct sunlight (e.g. http://www.ucar.edu/learn/1_5_2_23t.htm), the recommended use of high-visibility materials in poorly lit situations (e.g. http://www.ipmba.org/reviews/The_Nee...ty_Apparel.pdf ) indicate that there is enough uv/light to bring about an "almost magical" fluorescence also at dusk and dawn.

Could you point me to an article or two that helps explain away this apparent paradox? or a thread in the forum, at that. This one, e.g., was close to answering my question... https://www.physicsforums.com/archive/index.php/t-123480.html

 

[Greg Bernhardt]

Hello,

Thank you for bringing up this interesting topic. As a physicist, I can offer some insight into the phenomenon of fluorescence and its relationship to UV light.

First, let's define fluorescence. Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. In simpler terms, certain materials have the ability to absorb UV light and then re-emit it as visible light.

In the case of high-visibility materials, they are designed to absorb UV light and emit it as a bright, visible color. This makes them useful in poorly lit situations, as they can reflect light and make the wearer more visible. However, the amount of UV light present at dusk and dawn is not enough to cause fluorescence on its own. So how do these materials still appear fluorescent at these times?

The answer lies in the concept of the "Magic Hour." This is the time just after sunrise and just before sunset when the sun is low on the horizon and its light must pass through a greater amount of atmosphere to reach the Earth's surface. This causes the blue and violet wavelengths of light to scatter, leaving behind more red and yellow wavelengths. This is why the sky appears red and orange during these times.

But what does this have to do with fluorescence? Well, the red and yellow wavelengths of light that are present during the Magic Hour also contain some UV light. This is enough to trigger fluorescence in high-visibility materials, making them appear bright and fluorescent even in low light conditions.

In summary, the apparent paradox you have noticed is not actually a paradox. The UV light present at dusk and dawn is not enough to cause fluorescence on its own, but when combined with the Magic Hour effect, it can still trigger fluorescence in high-visibility materials. I hope this explanation helps to clear up any confusion. If you have any further questions, please don't hesitate to ask.