Einstein’s Relativity-theories predict: length-contraction, time-dilation, transverse-Doppler-shift, gravitational-spectral-shifts, expansion-of-space, time-dilation-of-super-nova-light-curves...etc. This letter attempts to gain some insight into two of them in terms of wave-theory. For example, the observations of time-dilation of ‘super-nova-light-curves’ are currently considered as a confirmative-test of expansion-of-time; whereas it is shown here that the super-nova-light-curve can be viewed as a ‘gate-function’ in the ‘time-domain’; and it can be Fourier-transformed into frequency-domain; and it is this wide band of waves which travels in space and reaches us after millions of years. During its travel, when this wide-band of waves gets red-shifted due to any mechanism, including the tired-light-interpretation of the ‘cosmological-red-shift’, then its band-width shrinks in the frequency-domain; so when it is transformed back into time-domain we find that the duration of the curve has got increased. Thus, time-dilation of super-nova-light-curves is not an independent observation from the observations of the ‘cosmological-red-shift’ due to any mechanism. Similarly, the relativistic length-contraction with speed is also explained in terms of waves, with the help of Fourier-transform.