Quantum mechanical wave function predicts probabilities of finding a ‘particle’ at different points in space, but at the time of detection a particle is detected only at one place. The question is: how this place gets decided, and can be predicted. To seek answer to this, we assume here that a ‘particle’ has some “diameter”, in stead of being a ‘point-particle’ of mathematical zero dimension; and depending upon the relative velocity between this particle and observer, its “diameter” experiences ‘Relativistic length-contraction’. Then we Fourier-transform this ‘length-contraction’ in ‘space-domain’ into ‘spectral-expansion’ ∆ω in ‘frequency-domain’, and find that momentum of a particle can be expressed as: m v = h ∆ω / 2 π c, and de Broglie’s wavelength, λB = 2 π c /∆ω ; as was derived in [ref.1 and 2. In the ref-2 it was shown that: in fact it is the ‘expansion of spectrum’ in the frequency-domain, which is the physical-cause for the Relativistic length-contraction.] Then we notice that the frequency-domain translation of the particle’s length in space-domain has a continuous spectrum; i.e. it contains a set of frequencies ranging from ωmax to ωmin . Therefore, as we found in ref. [3], this wide set of waves coherently add only at discrete points in space, and mutually nullify their amplitudes at rest of the places. And the place at which all the spectral-components of the wide band of waves will add constructively, will depend on the relative phase of all the spectral components. It is proposed here, that we need to know the relative phase angles of every spectral-component contained in the wide set of waves contained in the expanded wide band, for predicting the exact place of detection of the ‘particle’.