F 2 , ▁r ut ile ▁Ti O 2 ), ▁ne ▁> ▁n ▁o ▁and ▁thus ▁the ▁extraordinary ▁axis ▁( opt ic ▁axis ) ▁is ▁the ▁slow ▁axis . ▁ ▁Any ▁phase ▁ret ard er ▁with ▁fast ▁axis ▁equal ▁to ▁the ▁x - ▁or ▁y - axis ▁has ▁zero ▁off - di ag onal ▁terms ▁and ▁thus ▁can ▁be ▁convenient ly ▁expressed ▁as ▁▁ ▁where ▁ ▁and ▁ ▁are ▁the ▁phase ▁off sets ▁of ▁the ▁electric ▁fields ▁in ▁ ▁and ▁ ▁directions ▁respectively . ▁In ▁the ▁phase ▁convention ▁, ▁define ▁the ▁relative ▁phase ▁between ▁the ▁two ▁waves ▁as ▁. ▁Then ▁a ▁positive ▁ ▁( i . e . ▁ ▁> ▁) ▁means ▁that ▁ ▁doesn ' t ▁att ain ▁the ▁same ▁value ▁as ▁ ▁until ▁a ▁later ▁time , ▁i . e . ▁ ▁leads ▁. ▁Similarly , ▁if ▁, ▁then ▁ ▁leads ▁. ▁ ▁For ▁example , ▁if ▁the ▁fast ▁axis ▁of ▁a ▁quarter ▁wave ▁plate ▁is ▁horizontal , ▁then ▁the ▁phase ▁velocity ▁along ▁the ▁horizontal ▁direction ▁is ▁ahead ▁of ▁the ▁vertical ▁direction ▁i . e ., ▁ ▁leads ▁. ▁Thus , ▁ ▁which ▁for ▁a ▁quarter ▁wave ▁plate ▁yields ▁. ▁ ▁In ▁the ▁opposite ▁convention ▁, ▁define ▁the ▁relative ▁phase ▁as ▁. ▁Then ▁▁ ▁means ▁that ▁ ▁doesn ' t ▁att ain ▁the ▁same ▁value ▁as ▁ ▁until ▁a ▁later ▁time , ▁i . e . ▁ ▁leads ▁. ▁ ▁The ▁special ▁expressions ▁for ▁the ▁phase ▁ret ard ers ▁can ▁be ▁obtained ▁by ▁taking ▁suitable ▁parameter ▁values ▁in ▁the ▁general ▁expression