Figure 2

A space­time diagram illustrating negative velocity. Outside the caesium cell, light travels at c (indicated as a 45° line on the diagram). The small negative velocity in the cell means that light travels in the opposite direction. For the pulse inside the cell to match the peak entering the cell from outside, it actually needs to be emitted from the far side of the cell long before the peak arrives. At the same time, the transmitted peak leaves the far side of the cell. In other words, the transmitted peak appears long before the incident peak even enters the cell; this is what is meant by negative velocity.

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