Interesting facial expressions here. The closest match I can come up with is "It's gone," with its mix of pain, relief, joy, and fear. I've interspersed some of those frames farther down the page. They're not perfect matches, of course, but the similarities are noticeable - especially in the eyebrow acting.
As far as the reactions of other [movie] characters upon first seeing Gandalf the White, Frodo's is closest to Aragorn's. Since Frodo's not in that scene, I probably won't be screencapping it, but I love Aragorn's dumbfounded reaction while Gimli and Legolas bow in veneration. I've read the opinion that Aragorn doesn't bow because he's the king, but IMHO he's just so completely flummoxed that he doesn't even think of it.
Frodo wants to believe. To frame 10, he seems to come closer to doing so. But that's the turning point; after that things go the other way - a more questioning expression, his body sinks back down on the bed (although he still leans forward, as if to get a closer look), we gradually see more eyebrow action until it's close to the "It's gone" level. His "Gandalf" isn't an exclamation or even a statement - it's a question. Maybe a hallucination won't respond?
This is a closer camera shot, but still allows for cropping.