At first glance, *ẓill* means the shadow cast by objects when light falls on them. As the sun moves through the sky, the shadow changes from dawn until sunrise, from sunrise until noon, from noon until sunset, and from sunset into the night.
Before dawn, the shadow is absolute darkness. As dawn breaks, that darkness gradually diminishes, and when the sun rises, the shadow vanishes from the horizon. But this diminishing cannot be understood as the shadow spreading or lengthening, since the shadow is growing smaller, not disappearing altogether. Consider the narration of Abū al-Jārūd from Imam al-Bāqir, peace be upon him:
وَ فِي رِوَايَةِ أَبِي الْجَارُودِ عَنْ أَبِي جَعْفَرٍ ع فِي قَوْلِهِ أَ لَمْ تَرَ إِلى رَبِّكَ كَيْفَ مَدَّ الظِّلَّ فَقَالَ الظِّلُّ مَا بَيْنَ طُلُوعِ الْفَجْرِ إِلَى طُلُوعِ الشَّمْس
*Tafsīr al-Qummī*, vol. 2, p. 115
In the narration of Abū al-Jārūd, Imam al-Bāqir was asked about the verse: أَلَمْ تَرَ إِلَی رَبِّکَ کَیْفَ مَدَّ الظِّلَّ وَ لَوْ شَاء لَجَعَلَهُ سَاکِنًا He said: “What is meant by *ẓill* is the interval between the first appearance of dawn and sunrise.”
If we understand the noble verse in light of this narration from Imam al-Bāqir, peace be upon him, then *ẓill* does not in fact mean shadow, but rather the light as it spreads: the light of dawn, which begins at daybreak and extends until sunrise. In this sense, it is the shadow of light.
Now, if we interpret the sun as the first light of existence, its *ẓill* could be understood as the emergence of the world of existence from that first light…
God knows; He knows best.