The essential points worth discussing here are as said previously that the light intensity I ∝ ∝ E 2 ( r → → , t ) ¯ ¯ of the laser produces a proportional local potential which the atoms "feel" in that region. Furthermore, now we can tell the sign of such potential. We see that it follows the sign of Δ Δ ¯ ¯ which in turn follows the sign of the detuning. This implies that the potential is attractive if we have a red detuned laser ( ω ω < ω ω 0 ), and it is repulsive if we have a blue detuned laser ( ω ω > ω ω 0 ).