It's believed that there are three types of evil eyes
The first are unconscious evil eyes. These harm people and things, without intending to. The second type intends to harm. The third one is unseen, hidden evil which is the most scared one.
It was believed that, this eye saw all the wickedness in the world and removed poverty and ignorance. When Horus opened its eyes the world was enlightened, when he closed, it became dark. From Egypt, the eye talisman had spread to the Mediterranean, the Middle East, and Europe. When anyone looks at what is excellent with an envious eye he fills the surrounding atmosphere with a pernicious quality, and transmits his own envenomed exhalations into whatever is nearest to him. The glass reflects the evil intent back to the onlooker. It somewhat resembles an eye and it is said the blue colour is a factor in protecting the user.