In the classic experiment, light impinges on half-silvered mirror, the mirror reflects just half the light and transmits the rest. The photon before encounter with the mirror is [A>, then afterwards it evolves according to U to become [B> + i[C> (where C is the reflected light). The imaginary factor "i" is said to arise because of a net phase shift by a quarter of a wavelength which occurs between reflected and transmitted beams. I'm confused with this last part- Why, again, is "I" used? I'm unclear about the phase shift by 1/4 wavelength. Can anyone explain this in laymens?