The spindle fiber iteslf grows longer. The chromosomes move until they have seperated into two groups at the poles of the spindle.
Anaphase ends when the chromosomes stop moving.
Images of cells in anaphase:
Chromosomes, stained black, are pulled apart by spindle fibers.
In this image of a cell in anaphase, the spindle fibers are stained green, chromosomes are stained blue, and actin is stained red.
Biologists still do not know what seperates the chromosomes during anaphase.
Once anaphase is completed, the cell moves into telophase.