In the first part the author reviews the classical theory of the Fabry Perot interferometer that is applicable when the interferometer is illuminated by a light beam of coherent vibrations having long duration. In the second part, he studies how a pulse of square form is transmitted through a Fabry Perot interferometer. The two parameters of duration τ and τ' that characterize the instrument were defined. τ represents the duration of light path for a forward and backward motion between the two plates and τ' characterises the duration of the exponential attenuation of the beam due to the successive multiple reflections it suffers. In the case of the pulse having duration Θ0 > τ the emergent vibrations superpose progressively and the profile of intensity of the output pulse as a function of time is calculated. In case when Θ0 < τ the successive separated pulses do not overlap. The Fourier analysis of the signal received by a detector has been discussed in this case. In part III, the analogy between the spectrum of the signal and the optical spectrum of the Fabry Perot has been shown. In part IV, the profile of the light reflected by the Febry Perot receiving a light pulse, in the time domain, has been established and in part V the balance of light energy stored in the Fabry Perot has been established.