Lens

Lenses are an uncommon drop from Demon Eyes, but they can also be found in chests.

A player can combine 2 to craft Goggles, or 6 to craft a Suspicious Looking Eye to summon Eye of Cthulhu. Crafting the Suspicious Looking Eye also requires that you be standing near a Demon Altar

Another form of lens, called the Black Lens, can be used to craft Sunglasses. Black Lenses have a much lower drop rate than the standard Lenses.

As of 1.0.5, a single Lens is also an ingredient in the new Archery Potion. Also, the number of Lenses required to summon the Eye of Cthulhu was reduced from 10 to 6.