Many images of Orpheus and Eurydice show the couple right at the moment when Eurydice is yanked back to the Underworld forever. Orpheus stands at the edge of the cave, reaching out toward his wife as she is sucked into the darkness below. Totally tragic.
The division between light and dark is packed with ideas about life, death, happiness, and sadness. When Orpheus enters Underworld, Virgil describes it as a place of "great darkness." In addition to meaning depressing and gloomy, he also means that the Underworld is, quite literally, very dark. It's hard to make out shapes, and everything has sort of a dull, dead color to it. Thus, the absence of light corresponds to the absence of happiness. Bummer!
Then, as Orpheus and Eurydice approach the upper world, Orpheus sees the light of day. Literally and figuratively, this is the light at the end of the tunnel – offering hope and optimism to the weary couple. But when Orpheus turns around, Eurydice must return to the darkness and all of the misery it symbolizes. "With enormous night I am borne away," she says, again according to Virgil's Georgics.
The fact that this separation occurs at the threshold gives some awesome climatic tension to the story. If Orpheus had turned around half-way through their journey, it wouldn't have been nearly as gut-wrenching, don't you think?