Headcanons: The Physicality and Sensory Perceptions of the Nazgul
“They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys, and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared. And at all times they smell the blood of living things, desiring and hating it. Senses, too, there are other than sight or smell.” --Fellowship of the Ring, "A Knife in the Dark"
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As discussed in an earlier headcanon on the physicality of the wraiths, while in the depths of Sauron’s control they were invisible to normal sight, though not physically intangible. They existed in a sort of half-way state, partly in the wraith-realm or shadow-realm and partly in the "real" physical world.
While existing in this state, the senses of the Nazgul functioned very differently. By daylight, they saw only as through a glass, darkly. Their world was a grey realm in which living minds were patches of deeper grey. Thinking beings made, as it were, nearly physical impressions on the grey fog of their world, the weight of consciousness warping that otherwise little-differentiated reality out of shape. Only at the brightest part of the day were they blinded entirely, but they relied at all times quite heavily upon the vision of their living mounts.
By night, however, their vision was sharper than any normal man’s, sharper than a cat’s. Darkness revealed the world to them, rather than hiding it, for in the darkness living minds seem almost to glow and can be seen from far and far away.
But all of their senses were enhanced by their state, not least their sense of smell. It was not only blood that the Nazgul could smell so strongly; it was everything. Their bodies were altered to fit the needs of their Master, to make of them hunters who could not easily be outrun. They smelt and heard and saw as strongly and as well as any beast of prey, for their Master who designed them picked and chose carefully among all the most dangerous creatures of the world in gifting his servants with their new forms.
And now that they no longer stand straddling the worlds of shadow and of light, many of these gifts remain, their bodies permanently altered despite that they are visible now. The reason that the red eyes of a Nazgul seem to glow, especially at night, is that there is now a tapetum lucidum included as part of their eyeballs’ structure. Thus, their night vision is still extremely strong, though now they can see also in the daylight more like a normal man might do. Their hearing and their sense of smell, too, are still enhanced. It has been a great aid to Uvatha that he might run as one of his wargs, seeing and hearing and smelling just as one of them might.
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((NOTE: Despite the fact that I have spoken throughout this headcanon of "they," this information applies only to Uvatha unless otherwise stated by the other NazgulMuns. Much of this, in fact, will NOT apply specifically to Murazor the Witch-King, as his body was reformed by his ring after his "death" at Pelennor and is therefore now quite fundamentally different from those of the other Nine.