Dalek eyestalk

The Dalek eyestalk, also called the eyepiece, was the part of the Dalek which enabled the Kaled mutant inside its shell to see.

Design
The Dalek eyestalk was mounted on the Dalek's dome and enabled the Dalek to see its surroundings. It was the only vulnerable part of the Dalek. (TV: Resurrection of the Daleks)

The Daleks produced by the Progenitor had radically redesigned eyepieces with black rings on the stick, fins on the cone, and the lens being organic rather than robotic. The viewpoint from this eyepiece is similar except for an opaque orange perimeter with honeycomb circuitry and protruding organic veins. (TV: Victory of the Daleks)

Abilities
With a wide-angle lens, the eyepiece could magnify images so that the Dalek could see across great distances and also track a person's footsteps. (TV: Destiny of the Daleks) The eyepiece was the most vulnerable part of the Dalek. Dalek eyestalks from a post-Last Great Time War era were capable of even greater forms of magnification, with various scales displayed in-vision.

The viewpoint from the eyepiece varied over time, from a simple telescope-like view (TV: The Dalek Invasion of Earth) to an infrared scanner (TV: Destiny of the Daleks) to a bright green (or white, in the case of Imperial Daleks) lens, with Kaled script readouts displayed in the top and bottom left corners. (TV: Remembrance of the Daleks) By the time of the Last Great Time War, the eyestalks all contained blue glowing lenses, with a targeting aid "on-screen" at all times. (TV: Dalek)

Weaknesses
If the eyepiece was destroyed or if the lens was covered, the Dalek became a reduced threat. Daleks would behave erratically under these circumstances as the creature inside the casing was unable to deal with any danger. They would move around uncontrollably and fire their weapons in a blind panic, usually accompanied by the frantic screaming of "My vision is impaired! I cannot see!". (TV: The Daleks, Resurrection of the Daleks, Revelation of the Daleks, The Parting of the Ways)

Whilst on Skaro, in order to escape from the Dalek base, the Fourth Doctor threw his hat onto a Dalek's eyestalk. The Dalek began firing incandescently, guided by Davros, and almost hitting Davros in the process until the Doctor destroyed it. (TV: Destiny of the Daleks)

During the Shoreditch Incident, Ace fired an ATR (Anti-tank rocket) at an Imperial Dalek and was shocked at the damage. She responded by saying "I aimed for the eyepiece". While inside a Dalek shuttle craft, the Seventh Doctor merely placed his hat on a Dalek's eyepiece to confuse it while he "shorted it out". (TV: Remembrance of the Daleks)

However, post-Time War Daleks overcame this exploitable weakness. Their shielding could resist most bullets, although bastic bullets were still somewhat effective. (TV: The Parting of the Ways) They could also melt any hindrance, as Wilfred Mott discovered when he shot a Dalek eyestalk with a paintball gun. The paint was immediately melted off and the Dalek was heard to say "My vision is not impaired". (TV: The Stolen Earth)

The Daleks of the New Dalek Paradigm had the ability to repair their vision if their eyestalk was damaged, so the weakness could only be exploited as a temporary distraction. (TV: The Big Bang, GAME: The Eternity Clock) However, repairs could not be made if the eyestalk was severed from the main casing or if the damage was too great. (COMIC: The Only Good Dalek, The Dalek Project)

Other uses
Dalek eyestalks were worth a considerable amount during the Second Dalek War of the 26th century. They were collected by Dalek Killers and bounty hunters and were worth 20 decacredits after they were returned to the Earth forces to confirm a kill. Some people like Jon Bowman and his crew on-board the Wayfarer did this for a living. (PROSE: Love and War, Prisoner of the Daleks) Outside of times of war, eyestalks still had value and could attest to someone's character since killing a Dalek was no small feat. The Doctor in his eleventh incarnation once salvaged the eyestalk of a damaged Supreme Dalek to use as bartering leverage with Gantok to learn more about the Silence. (TV: The Wedding of River Song) On another occasion, the Doctor used an eyestalk to display proof of his courage and comradeship to gain the trust of one of the many fleets besieging Trenzalore, before realising he had landed on a Dalek ship. (TV: The Time of the Doctor) He kept the eyestalk of a Dalek from the Time War in the TARDIS drawing room. (GAME: TARDIS, The Gunpowder Plot)

While on Skaro, the Eleventh Doctor sent Amy Pond to salvage a Dalek eyestalk, which he needed to create a device to blind the rest of the Daleks. (GAME: City of the Daleks)

The Daleks' humanoid puppets also had eyestalks, which emerged from their foreheads. (TV: Asylum of the Daleks, PROSE: The Dalek Generation, TV: ''The Time of the Doctor)

The robotic Dalek that the Doctor dubbed Quasimodo Dalek had two eyestalks, both in place of its gunstick and manipulator arm. A gunstick was attached where the eyestalk should have been. (COMIC: The Dalek Project)