I was reading Stamper-Kurn's article Experimental Methods in Ultracold Atomic Physics (link). In the imaging section (page 13), he mentions:
Cold atoms are conventionally probed by optical imaging. Probe light at a well deﬁned optical frequency is sent through the atomic gas and imaged onto a camera.
What determines this "well defined optical frequency"? For example, why would one choose 780 nm, which seems to be a common wavelength?