Open the Iris

Normally, the eye filters light by widening or closing the hole in the centre of the iris, called the pupil. Emotional reactions can achieve this as well: shock might narrow the pupil, while relaxing could widen it considerably. However, in animated fare or Comic Books, this can be very hard if not impossible to show, since a typical human pupil, at its widest, is 3-4 millimeters. This trope is a way to sidestep that difficulty. Instead of just changing the size of the pupil, you change the size of the iris (the colored ring surrounding the pupil) as well.

Your iris is no more capable of growing than your bones are of stretching like Gumby, but since they are by far the largest feature of an eye other than the whites, it's easy for artists to "expand" them to get the desired emotional reaction across.

The opposite, when the irises are drawn in a shrunken state to indicate fear, pain, or shock is Wide Eyes and Shrunken Irises. Compare Expressive Mask.

When the pupils are drawn abnormally large, the eyes generally appear pleading or wistful. That trope is Puppy Dog Eyes

Opening the iris in Stargate SG 1 has nothing to do with this trope.

Anime and Manga
"Kageyama: "... you like Takasaki-san, don't you!?"
 * The animation style of Magical Girl Lyrical Nanoha usually shows an all-black pupil/iris with the anime-standard lens flair and a cresent of color near the bottom of the eye. When a character is shocked, scared, or incredibly pissed off, you can see the iris all the way around. Their eyes kind of shrink, too.
 * Since just about every main character in Peacemaker Kurogane spends a lot of time indulging in Wangst over past and/or current traumatic events this happens often. (The most notable characters who do this are Tetsunosuke and Suzu.)
 * Name an anime. Go on. When the Moe Blob cries her irises will become HUGE.
 * Also used for expressing joy, when this trope is played for laughs often they iris fills the entire eye (no whites), with the eyes also given an odd shape, like wavy lines.
 * Done in many Reaction Shots in Serial Experiments Lain.
 * Similarly used in Ghost Hound, by the same director.
 * In a particularly extreme example, in Magikano, the iris opens, then the iris and pupil shrink, and the sclera fractures.
 * This is used along with Hellish Pupils in Higurashi no Naku Koro Ni - probably because.
 * Train from Black Cat does this a LOT. Though, even for anime standards, his irises were already not normal, looking a lot more like a cat's. Creed also does this sometimes.
 * It happens occasionally to Sousuke from Full Metal Panic, though not too often, considering how normally unphasable he is. He's shown doing this more often during TSR, and it's noticeable in the Sigma manga as well.
 * People in Gundam Seed who enter SEED Mode have much larger irises, though their emotional range typically doesn't change much.
 * In Naruto, this is what the 9-Tailed Fox looks like when.
 * Baby Steps: This technique is used to reflect Eiichirou's shock when Kageyama suggests he knows the reason E-chan got into tennis.

Eiichirou (his irises shrinking to tiny dots): "EEEEEEEHHHHHH!?!?!"

Kageyama: "What's with that reaction?""

Film

 * Tweaked in 2004's Dawn of the Dead 2004. When the anonymous woman reanimated, we got a closeup shot of her eye turning from normal human green to that freaky occluded color.
 * Happens a lot in horror movies, when a person who's a vampire/possessed/under a spell goes from looking normal human to looking like they're a vampire/possessed/under a spell.

Literature

 * In J.R.Ward's Black Dagger Brotherhood paranormal romance series, the vampire seer character Vishous has this happen to his left eye whenever he deliberately tries to look into someone's future or to bury their memories.

Music

 * A prominent visual in Lady Gaga's video "Bad Romance."

Webcomics

 * Dominic Deegan features irises getting huge and limpid when someone is feeling touched or romantic toward someone else.
 * Truck Bearing Kibble in this strip
 * Happens to Kate Beaton when she's traumatized by James Joyce's love letters.
 * In Blip, Hester's irises are massive by default, larger than any other characters' (it's even Lampshaded early on); a visual indication that she's the most emotional of the bunch.
 * In the end-of-Act-5 flash, Rose comes to a sudden realization about the nature of The Tumor. Her eyes widen as the camera zooms in on them for dramatic effect, and then everything goes wrong.

Western Animation
"Sokka: Driiiink cactus juice! It'll quench ya! Nothing's quenchier! It's the quenchiest!"
 * Common trope in Western Animation, especially happening to children when they are surprised by something (to the perspective of a kid) mortifying. Used at least once in nearly every Nickelodeon and Cartoon Network show.
 * Common trope in The Fairly Odd Parents.
 * Avatar: The Last Airbender
 * "The Desert"

"Long Feng: The Earth King has invited you to Lake Laogai.
 * "Lake Laogai"

Jet: I am honored to accept his invitation."


 * In Kim Possible: So The Drama, Kim's iris and pupil go bigger, then smaller when she is shocked.
 * The Spectacular Spider Man plays with the trope. Curt Connors' iris narrows to a vertical slit, then opens back up to a normal human one. Well, sort of. None of the people in this Animated Series have pupils, just iris.
 * In The Simpsons, this is one of Homer's reactions as he chows down on Krusty Burger's barbecue sandwiches, repeated along with the action itself and fat and suchlike flooding into his system.
 * Also, when he licks hallucinogenic toads.
 * In general, this is the way Simpsons characters show that a psychoactive drug/medication is kicking in. Futurama uses the trope in the same way.
 * My Little Pony Friendship Is Magic provides the page image. The animation uses eye tropes heavily to convey emotions, and Open the Iris is used to show, just for example, epiphany, wonder, greed, insanity, terror, emphasis of a key phrase, and the sensation of burping up a scroll.
 * In SpongeBob SquarePants, Gary gets this when Spongebob informs him it's time for a bath...though in his case, his pupils growing bigger means he's scared.
 * Happens to Bruce in Finding Nemo as a result of him suddenly reverting back into "a mindless eating machine" after accidentally sniffing Dory's blood.