Visible light is the part of the electromagnetic spectrum that can seen by the human eye. As shown in the chart below it's wavelength ranges from 380nm (nanometers) to 750nm. A nanometer is one billionth of a meter. Some birds and insects can see wavelengths of light that are even shorter. This portion of the spectrum is known as ultraviolet light.
Ultraviolet light has a wavelength of 10nm to 400nm. We're interested in the the 100nm to 400nm range which is more commonly known as UVA or Black Light. Certain materials when exposed to Black Light will glow with visible light.
To Find The Cache
At the posted coordinates you will find a tree with a reflector on it. Search for a small container there. Inside you will find a small piece of paper. Use your ultraviolet light to reveal the west coordinates of the cache.
Now head down the south trail. Within 150m you will find another tree with a reflector. Search for another small container. Use your ultraviolet light again on the contents to reveal the north coordinates of the cache.
Now go find it! Don't forget to get the code for the BFL V final. Despite what it appears, I didn't forget to write it down.