Samples are centrifuged in cells which are transparent top and bottom. A laser scans the cells at regular intervals during centrifugation, recording changes in absorbance along the cell. A typical result from a sedimentation velocity experiment is shown at top right above. Early scans (in blue) show the boundary moving as molecules start to sediment; later scans (in red) show the molecules have been cleared from the top of the cell, giving a low absorbance, and accumulating at the bottom of the cell. Larger molecules (e.g. multimers) will tend to sediment faster than smaller ones; globular, folded molecules will sediment faster than asymmetric or unfolded molecules of the same molecular weight. Different rotor speeds are chosen for analysis of different sized molecules.