The light before night

This original descriptive passage is set in an invented lighthouse at dusk.

At four in the afternoon, the lantern room looked less like the top of a lighthouse than a cupboard surrounded by windows. A tin of polish stood open beside a folded cloth. The brass fittings were dull where the keeper's fingers had touched them, and three dead flies lay on the white-painted sill. Far below, the sea was still plainly divided into blue water, grey rocks and the narrow white threads of waves.

The keeper worked around the room without looking hurried. She cleaned salt from each pane and tested a handle that moved only a few centimetres. Her actions were so small that they seemed unequal to the tower beneath her. She entered a time in the logbook, drew one firm line under it and waited. Nothing in the room yet explained why the lighthouse had been built so high.

Then the distance began to close. The far headland lost its cottages first; next, the fields above the cliffs became one dark strip. A fishing boat that had been easy to follow was absorbed into the water, leaving only a brief mark of foam. Day had allowed the keeper to watch the sea. Night appeared to be asking the lighthouse to answer it.

She switched on the lamp. At its centre, the light was no larger than a clenched hand, but the curved glass gathered it and sent a bright blade across the room. Dust flashed in the beam and vanished. As the lens began to turn, the keeper's shadow travelled around the walls, stretched across the windows and was swept away. The ordinary objects changed with every revolution: the cloth blazed white, the logbook opened like a pale signal, and the brass handle briefly held a line of fire.

Outside, the beam crossed water that no one in the room could now examine. For a moment it lit the wet cliff, then moved out towards vessels hidden beyond the dark. The keeper could not see anyone receive it. She listened instead to the steady mechanism and made a slight adjustment to the handle she had tested earlier. That modest movement sent the light farther across the sea.

When she left, she carried the polish and cloth down with her. The room was almost empty again, but it no longer looked unimportant. Above the closed door, the beam continued its measured return, offering the same brief assurance to people whose names the keeper would never know.

How does the writer make the approaching darkness transform the lighthouse from an ordinary workplace into a source of reassurance? Support your answer with references to the passage.