There are many things that can cause your eye to turn red.
The eye looks red when the blood vessels that are in the conjunctiva (the mucous membrane that covers the white of your eye and the backside of your eyelids) becomes dilated.
Those blood vessels often dilate when the eye gets irritated. This irritation can originate from a problem occurring inside the eye or factors from outside the eye.
The most common external factors that can cause the eye to become red are exposure to...