This applet shows what happens when atoms wander
into the paths of four lasers (two in each direction).

Adjust the frequency of the laser until it acts like "optical molasses" and traps the atoms in the center. Explanation: The lasers will only interact with the atoms at certain frequencies, indicated by the peak of the curve (called "resonance") in the lower right. When the lasers are right at resonance, they cancel each other out and the atoms feel no net force. I.e., they are being "squeezed" from both directions but not slowed down. Doppler shifting due to the motion of the atoms, however, means that each atom will "see" a different frequency based on what direction it is moving. Making the frequency a little lower than resonance will cause atoms moving toward the laser to "see" the resonance frequency. Making the frequency a little higher will cause atoms in the lasers to speed up.