Laser Eye

The Sentry Eye is an eyeball which attacks Link and guards certain areas.

A Link to the Past
In A Link to the Past, the Sentry Eyes are found in Misery Mire, Turtle Rock and Ganon's Tower. They block access to certain areas and pelt Link with laser beams. The only way to stop their attacks is to acquire the Mirror Shield, which will allow Link to progress further into the dungeon. Some eyes only attack when Link looks in their direction, while others will attack whenever he walks into their line of sight. To bypass the ones which attack when he looks at them, Link must charge up his sword and walk backwards past them.

Ocarina of Time and Majora's Mask
In Ocarina of Time and Majora's Mask, the sentry eye appears to be a silver Eyeball Switch, but instead of opening immediately, it remains closed until Link draws near, at which time the odd eye will open and shoot a fireball that slowly chases Link once it touches ground. There is no way for Link to guard against this projectile other than avoiding it.