When an observer is moving towards an electromagnetic (EM) wave, a phenomenon called "Doppler effect" occurs, resulting in a change in the observed wavelength. The equation that represents the change in wavelength due to the observer's motion is given by:
λ' = λ(1 - v/c)
Where: λ' is the observed (shifted) wavelength, λ is the wavelength of the wave in its rest frame (source wavelength), v is the velocity of the observer relative to the wave, c is the speed of light in a vacuum.
In this equation, if the observer is moving towards the source (wave), the observed wavelength λ' will be smaller (decreased) compared to the source wavelength λ.
It's important to note that this equation assumes non-relativistic speeds, where v (velocity of the observer) is much smaller compared to c (speed of light). For relativistic speeds, a more complex equation, which includes the Lorentz factor, would be necessary.