Perhaps a little more prosaically, I'd like to propose that there are two kinds of 'light' phenomena going on here.
First, Goldberry comments that Frodo has a light in his eyes and a ring in his voice. Perhaps this is simply a 'glint in the eye' and a vivacious personality being described, different from most of the 'dull' hobbits.
Secondly is the shining of 'inner light', which many have proposed as indicating Frodo's spiritual purification during his quest. I think that Frodo is gaining an increased presence in the 'spiritual realm'. In this he is slowly starting to approach the radiance of Glorfindel, revealing his great spiritual power on the 'other side'. Naturally this is not as extreme as an elf lord revealed in his anger!
I'd think that this spiritual power may be greater in those elves who have been to Valinor. This could fit with the power and wrath that Feanor and co. displayed on their return to Middle Earth.
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Rumil of Coedhirion
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