[Lalwendë]

The simple explanation is that light shining through branches is one of the most beautiful things you can witness (apart from when driving) as it shifts, patterns are created, sometimes even forming fleeting images, and when coupled with glass or water droplets, rainbows can be seen moving about. I am sure Tolkien is not one of the first to have loved this sight. As for hair, shiny locks are a traditional signifier of beauty.

But in Tolkien's case, light has become Light, especially in Valinor. The light seen in Middle-earth is possibly just good old light, but transformed by the movement of trees and hair it becomes an altogether more beautiful thing and could easily become Light. The ancients of the primary world revered light as seen through Trilithons and openings in tombs (the midwinter sun at Newgrange for example) - for these people too, light became Light.

It fascinates people and always has done. This brings to mind again Newton's own strange experiments to find out exactly what the nature of light was, and likewise, Saruman's own fascination with Light. Could it be that there was more to it when he cut the trees down around Orthanc?

Light through trees in Middle-earth could be viewed by the people as something approaching the divine. Especially for Elves who had been in Valinor, such as Galadriel, it may have conjoured memories of the Two Trees, and for those who had not seen them, simply magical experiences which they interpreted as divine.

It could also have much to do with Yggdrasil, but I shall have to look this up a little more...