The Torah says to set a "tzohar" in the ark — a mysterious word usually translated "window" or "light." Targum Pseudo-Jonathan on (Genesis 6:16) tells us Noah had to fetch it.
"Go thou unto Phison, and take from thence a precious stone, and fix it in the ark to illuminate you."
Pishon is one of the four rivers that flowed out of Eden (Genesis 2:11). Noah is instructed to travel to the edge of paradise, retrieve a luminous gem, and install it inside the ark. The ark's light source is literally a piece of Eden.
The light that would survive the Flood
The Targumist's image is haunting. The sun will be blotted out for forty days and nights. Rain, darkness, the waters above and below reunited. In that total blackness, the only light in the only surviving vessel comes from a stone taken from the gates of paradise.
Jewish mystical tradition develops this further. The stone is sometimes identified with the shamir or with the glowing gems that will later light Solomon's Temple. In the Targumist's hand, the point is simpler: even during the destruction of the world, a splinter of Eden's light is preserved in the ark. And where the light is preserved, humanity can be preserved.
"With the measure of a cubit shalt thou complete it above. And a door shalt thou set in the side of the ark; and with dwelling-places, inferior, second, and third, shalt thou make it." Three stories — one for humans, one for animals, one for waste, by later tradition. A precise, livable ark under a single glowing stone.