  Optical propagation time in matter could reveal fruitful information, such as
the velocity of light and the sample's refractive index. In this paper, we
build a simple and robust setup for measuring the optical propagation time in
matter for a known distance, the system uses high frequency square signal as
the signal carrier, and a lock-in amplifier is employed to obtain the phase
difference between the reference square signal and the other one penetrating
the sample, in this way the optical time of flight in matter can be obtained by
a background subtraction process. Primary experimental result confirms the
feasibility of the newly proposed measuring theory, which can be used to
measure easily in high-speed the speed of light and the refractive index of
optical transparent material, compared with the currently popular measuring
technique using oscilloscope, potential advantage of our proposed method
employing lock-in amplifier is that high accuracy are promising, and in
contrast with the presently most popular method for measuring the sample's
refractive index based on the minimum deviation angle, superiority of our
suggested method is the easy preparation of the sample, the convenient
operability and the fast measuring speed.
