S. R. Faruqi:

With changes in the light source, the glistening of the sand-grains diminishes and increases; in the glistening of a sand-grain there's a kind of flickering/trembling as well. On this basis, the speaker has supposed the sand-grains to be restless/agitated. There can be two reasons for the restlessness of the sand-grain. Perhaps the sand-grain is restless out of sympathy with the oppressed one who, having come into the beloved's street (or died), has become mingled with the dust. Or else it is restless because when the oppressed lover mingled with the dust, his restlessness affected even the sand-grains. The second cause is better. The verb ral gayaa is fine, and brings the verse close to ordinary life.

But Ghalib has taken this theme much further:

G{29,1}.

Firaq Sahib has, as usual, lowered Mir's theme; the address to the street of the friend is unnecessary and ineffective:

dil jale ro))e hai;N shaayad is jagah ai kuu-e dost
;xaak kaa itnaa chamak jaanaa ;zaraa dushvaar thaa

[heart-burned ones have perhaps wept in this place, oh street of the friend
for the dust to sparkle so much was a bit difficult]

FWP:

SETS
MOTIFS == ROAD
NAMES
TERMS

It's hard to find a proper English counterpart for ;zarrah . It means something like a 'grain' or 'particle'; In the ghazal world it's usually treated as a particle of dust; here, it's the kind of dust or dirt into which a corpse could be pulverized. And yet it's also made to glitter in the sun like a grain of sand. Which just goes to remind us that the ghazal world isn't bound by the laws of nature.

The dust might also be thought of as restless in the sense not of glittering but of blowing around constantly in the wind. That removes the problem of the glitter, but doesn't bring us any closer to the natural world, because a street in which 'every single' grain of dust was constantly blowing around would resemble a dust-storm or a series of dust-devils or some other bizarre scene; one would hardly be able to call it simply a 'street'.

The second line begins with two excellently suggestive little words. 'Here' reminds us that oppressed ones die and are mingled with the dust all over the place, so the speaker is only talking about one more example of a general situation. And 'which' makes it clear that no other explanation for the restlessness of the dust needs to be considered-- the speaker need only wonder 'which' of many possible candidates is the one whose pulverized body is agitating this particular stretch of street-dust. It almost seems that he's asking the beloved whose body she's disposed of lately; but of course, she might not have actively slain anybody, she might simply have happened to notice that one of her lovers wasn't around any more.