It was Director Alice Troughton & Cinematographer Dale McCready's idea to have Merlin keep his eyes open, but look asleep to Arthur, & not move till Arthur says his name. In much of Arthurian Legend, Arthur is associated with Apollo the Sun God. It was important to Alice Troughton that Arthur represent the Apollonian Arthur in this scene. So her & Dale McCready worked together to make that apparent with lighting, timing, Colin Morgan (Merlin) & Bradley James (Arthur). She says that on the whole, it was "a read team effort," & they were pleased with the outcome. It was also important to convey, not just through Arthur's line, that "it's a new day." In more ways than one, this new day brings a new king, & Arthur finally takes his rightful place in 'Destiny,' previously described in "Merlin" as a new world called "Albion." Though Albion may not have formed quite yet, this is a major marker on the path to getting there.
In my own interpretation of this scene, the fact that Merlin's eyes are open where Arthur can not see them represents the knowledge Merlin has About Arthur's destiny which Arthur is unaware of. Merlin can see Arthur's figure come through the door by means of Arthur's shadow, but Arthur doesn't know Merlin is awake. I feel that the shadow to Merlin represents all that Arthur is, and can be; now that Uther is dead. Merlin doesn't know if he will see the Arthur he hopes for when he turns to look at him. He lingers on the shadow not only because he feels guilty for his part in Uther's death and doesn't want to face Arthur, but also because the shadow seems to represent what Merlin sees in Arthur, a figure in destiny more important than Arthur himself may even know. The grand entrance of the shadow projects a simple and familiar silhouette, yet expresses the nexus of Arthur, the sun, and all they represent. Turning to look at Arthur was thus doubly significant since Merlin seemed to have to tear himself away from the auspicious shadow at the same time it was hard to face Arthur; you can feel the super-charged dichotomy in his physiology [nice work Colin Morgan]. The new day brings with it a new Arthur and a new beginning; Arthur watches Merlin for a moment after exiting a room which represents the past. He does not have to search at all for what the future will hold, for Merlin is already there in front of him, guiding him into it. "Merlin," and Merlin finally, apprehensively tuns to face Arthur; the future of Camelot has begun. Alice Troughton said that it was actually Colin Morgan who first spotted the shadow on the wall, I'm glad he did because it seems it was an integral part of what her and Dale McCready were trying to accomplish! They did it!
Nice work Merlin peeps!