"The light, it's strange out here...it's kind of like it doesn't scatter quite right."
"When light hits small particles the light scatters in all directions...scattered light then undergoes [an] interference by the surrounding particles...it can occur when light travels in transparent solids and liquids, but is most prominently seen in gases [such as the atmosphere]...scattering contribute[s] to diffuse light...dependent upon the size of the particles and the wavelength [or color] of the light." (Wiki)
So Dan could maybe have either noticed an odd color shift to the ambient island light (via the visible spectrum), or perhaps the kind of effect that you see during a solar eclipse. That's when the light of the sun passes around the moon on its way to the earth, and you can see in the light between the shadows of tree leaves a kind of diffused effect, like the shape of the sun during the eclipse. Magnetism and gravity can bend light also, creating odd visual results. Hope that we hear more about this soon! Thank you TPTB for sending a physicist to the island! :-D
More on the color spectrum and light scattering: