Another way of changing frequencies is to change the positioning of the laser, for example, by using a monochromatic (single-color) laser that has a frequency that is a little below one of the "resonant" frequencies of this atom (at which frequency the laser will not directly affect the atom's state). If the laser were to be positioned so that it was moving towards the observed atoms, then the Doppler effect would raise its frequency. At one specific velocity, the frequency would be precisely correct for said atoms to begin absorbing photons.