Some assume pirates cover one eye by using eye patches due to the injuries in battle and might be other reasons.There are many theories behind the reason.But in fact,the eye patch is more likely used to fight in the dark.It helps pirates without suffering from night blindness.These are some theories what are behind the reason.
Suppose if they are moving from a brighter region to a darker region,it is hard to adjust eyes to the dark.Averagely,it takes 25 minutes to adjust our pupil in dark.If we cover an eye with an eye patch for some time,the pupil will adjust to see in the dark.So at the time of battles during night time,they can use their covered eye to see in dark more quickly.
Another theory was frequent using of telescopes causes eye injury.so they use eye patches.
It was also a misconception resulted from fictional characters who often wear an eye patch,an injured leg or hand and a bird on shoulder.
