1. A system for treating an eye of a patient, the eye including a fluid and a tissue posterior to a cornea, the system comprising: a femtosecond laser configured for directing femtosecond energy pulses along an optical path; an optical system disposed along the optical path from the laser, the optical system including a focusing lens and a scanner so as to scan a non-plasma-generating focus of the femtosecond energy along a scan path within the patient's eye, the scan path extending posterior to the patient's cornea within the fluid and the tissue so that the scan path includes a first location disposed within the tissue of the eye and a second location disposed within the cornea; a sensor, comprising an image acquisition device, oriented and disposed along a part of the optical path outside of the patient's eye so as to sense a first signal to form a first reflectance image associated with a first focus location within the eye, and to sense a second signal to form a second reflectance image associated with a second focus location within the eye, the first and second signals being reflectance signals of the femtosecond pulses of the laser which have a wavelength equal to that of the femtosecond pulses, which are reflected respectively by the eye at the first and second focus locations; and a computing device communicatively coupled with the sensor, wherein the computing device determines a location of an interface between the fluid and the tissue of the eye based on a comparison between a size of a focal spot in the first reflectance image formed by the first signal and a size of a focal spot in the second reflectance image formed by the second signal.