Rule of Perception

If the audience can't perceive it, it doesn't exist.

The audience of a movie will know only what they can see and hear. This means that nothing really exists in a movie unless you can see or hear it (because if it does, you're going to have to explain why it was Behind the Black). As a result, a kind of accepted audio-visual shorthand has been created over many years, to help the audience understand what they're looking at and what's going on.

Tropes based on this phenomenon include:

Gems are naturally shiny and pre-cut so that the audience can recognize them. Characters who are more competent will also look older/more mature. You can't see air; therefore, it does not exist. The characters already know this exposition, but the viewers don't, so it has to be stated onscreen. It's easy to tell when a blade is sharp because it makes a "sharp noise." Computers beep so that you can tell they're doing something. Anything that's offscreen is invisible. There's only so many weird tentacles and extra eyes and such to add to a character design before they all blend together. At that point, if you want to make a clearly visible transformation, you have to remove all those weird features and go back to a humanoid design. Stationary images have to do something to represent motion. Bullets spark when they ricochet to make it obvious where they hit. You can tell this guy is using a special attack because he's shouting the attack's name while he's using it. Cheese is always immediately recognizable by a distinctive shape and color. Chainsaws are flashy and make a lot of noise, so they must be really powerful. If you can't be seen, you can't be hit. Fire and lava are only dangerous if you touch them directly. If a character has a seizure, the writers will make it the most visible kind of seizure. You can tell a character is terrified by their exaggerated cowering. The audience probably doesn't know about foreign cultural norms, so the characters will be clueless as well. If you don't touch the visible part of a tornado, the most it can do is whip your hair around a little. A clear, auditory representation of a character's shock. Non-visual senses like smell and pain have to have some form of visual representation. Fancy clothing is an easy way for us to tell which characters are royalty. Bullets leave visible trails in the air to make it easy to tell which way they went. Everything has a progress bar, even if it logically shouldn't, so the viewers know how close it is to being done. Computers use flashy, unnecessary graphics. You can tell a video game character just got damaged because they briefly flashed a different color. When you're tracking someone, their trail will glow brightly so the audience can see it too. Lasers behave in unusual ways that make them more visible. Fraternal twins need to be (nearly) identical too, or the audience won't recognize them as twins. There will always be an obvious cue so we know the exact moment when a character dies. Ninjas are stealthy, but they can't be so stealthy that the audience doesn't know they're there. As long as you don't touch the rocket, you'll be okay. It's dark, but not so dark that we can't see what's happening. Holograms have little flickers and static effects and such so that it's obvious they're not real. Physical blows make loud noises so the audience knows when someone gets hit. A type of security system that is conveniently easy to see. Snipers use laser sights so that the audience can tell where they're aiming. If it's not visually obvious that your superpowers are protecting you, you'd better state it explicitly in dialogue. The audience can't feel the character's pain, so the character won't be incapacitated by what should be serious injuries. When a character is bobbing up and down, you know they're floating in the air and not just misaligned with the background. When a character shapeshifts, there are visual cues that make it easy to tell they're the same person. If the writers want the audience to know the bad guy's motives, he has to actually explain them at some point. For the benefit of the audience, having characters narrate events that should be extremely obvious to them. How else will you know that it's foreign? The camera doesn't have peripheral vision, so neither do the characters. As long as you don't splat into the ground, you'll be okay. As long as a character is offscreen, it's assumed that they continue doing whatever it was we last saw them doing. Offscreen characters are in a sort of limbo that allows them to reappear wherever they like when they come back onscreen. As long as you can escape the visible blast, you won't be hurt by the invisible shockwaves that would tear you to bits in real life. Power is represented with a highly-visible glowing effect. A strictly-mental injury is represented with the more-visible effect of a nosebleed. Parachutes are small enough to fit on the screen. You can tell the voice is coming from the radio because it's slightly distorted. We can't hear the other end of his telephone conversation, but that's okay because he'll repeat it back for us. If it isn't visible, it isn't audible either. Why shouldn't water be soft? It looks pretty soft! A visible disruption effect in the air, usually with appropriate sound effects, accompanies a force field. A character reads something out loud for the benefit of the audience, even though there's nobody else around to hear. There is sound in space because the viewers want to be able to hear what's going on. An index of visual shorthands that help the audience understand what's going on. Poison is brightly-colored so it's easy to tell that it's poisonous. For obvious reasons, you can't just display a character's genitalia, so you've got to find some way to make it clear who's a boy and who's a girl. Actors exaggerate pain for the benefit of the audience. When a character is traveling through a pipe, the pipe will bulge to show their location. Computer interfaces are designed for the viewers watching them on TV, not the characters who are actually using them. The audience needs to be able to see what an invisible character is doing. We can't see the actual text of the letter, so instead we hear a voiceover of the person who wrote it. If characters swap bodies, their new body will talk in their old voice so that you can tell it's the same character. A character just entering the set has somehow heard what the characters already there were talking about before they walked in--the audience knows, after all, so the characters should too. There's no point in having it rain so lightly that the audience can't even tell it's raining, so if it's gonna rain, it rains a lot. When something is tunneling underground, you can tell where it is because it will displace dirt or break floorboards on the surface. You can tell he's being electrocuted because his skeleton is showing through his skin.
 * All-Natural Gem Polish
 * Age Is Relative
 * The Air Not There
 * As You Know
 * Audible Sharpness
 * Beeping Computers
 * Behind the Black
 * Bishonen Line
 * Briffits and Squeans
 * Bullet Sparks
 * Calling Your Attacks
 * Cartoon Cheese
 * Chainsaw Good
 * Concealment Equals Cover
 * Convection, Schmonvection
 * Convulsive Seizures
 * Cower Power
 * Culture Blind
 * Do Not Touch the Funnel Cloud
 * Dramatic Stutter
 * Editorial Synaesthesia
 * Ermine Cape Effect
 * Every Bullet Is a Tracer
 * Exact Progress Bar
 * Extreme Graphical Representation
 * Flash of Pain
 * Fluorescent Footprints
 * Frickin' Laser Beams
 * Half-Identical Twins
 * He's Dead, Jim
 * Highly-Visible Ninja
 * High-Speed Missile Dodge
 * Hollywood Darkness
 * Hologram Projection Imperfection
 * Kung Foley
 * Laser Hallway
 * Laser Sight
 * Luckily, My Powers Will Protect Me
 * Made of Iron
 * Mid-Air Bobbing
 * Morphic Resonance
 * Motive Rant
 * Narrating the Obvious
 * National Stereotypes
 * No Peripheral Vision
 * Not the Fall That Kills You
 * Offscreen Inertia
 * Offscreen Teleportation
 * Outrun the Fireball
 * Power Glows
 * Psychic Nosebleed
 * Puny Parachute
 * Radio Voice
 * Repeating So the Audience Can Hear
 * See No Evil Hear No Evil
 * Soft Water
 * Some Kind of Force Field
 * Sounding It Out
 * Space Is Noisy
 * Stock Visual Metaphors
 * Technicolor Toxin
 * Tertiary Sexual Characteristics
 * Theatrics of Pain
 * Traveling Pipe Bulge
 * Viewer-Friendly Interface
 * Visible Invisibility
 * Voiceover Letter
 * Voices Are Mental
 * Walk in Chime In
 * When It Rains, It Pours
 * Worm Sign
 * X-Ray Sparks

A lot of these violate Real Life physics because Reality Is Unrealistic. But Tropes Are Not Bad. Remember, this is in consideration to the audience, so if you're looking for realism, go check out naturalism, aka Slice of Life. The Rule of Perception is the whole reason Foley artists and sound mixers exist.

Related to The Coconut Effect, in that the Rule of Perception is often what causes the initial drift away from reality.

Anime

 * Detective Conan often has people whose actions the audience is suppose to know, but whose specific appearance is unknown to the other characters, rendered as all-black silhouettes even in places where there would be no shadows (even outdoors in the middle of the day).

Advertising

 * In a feminine-hygiene ad, some blindfolded women try to identify the rhinoceros standing between them. Their guesses are all inanimate objects, based entirely on the shape of its body parts that are seen on screen ("It's a rope", "It's a pillar", etc). None of them notice that it smells like a big freakin' animal, that it's warm to the touch, or that it's moving slightly.
 * A case of Rule of Perception meets Completely Missing the Point, as in the original parable that inspired this scenario (which involves an elephant), the blind investigators know it's an elephant, and are asked what the animal is like, not what it is.

Comic Books

 * Sue Storm's "invisible" self and force field are visible to the audience, by dotted outlines in the comics and Conspicuous CG Predator-like distortion in the movies.
 * Improvement in art quality has removed the infamous dotted lines from the comics and replaced them with the same effect used for glass.
 * In the Don Rosa story "The Three Caballeros Ride Again," Jose Carioca hides out in Donald Duck's trunk and asks him to help him flee from a bandit. The two exchange several lines of dialogue, but do not recognize each other until they are face to face--apparently in a comic book, neither of them can hear the other's distinctive speech patterns...
 * When Jean Grey/Marvel Girl of the X-Men uses telekinesis, visible (to the reader) pink energy is often shown moving from her head to whatever she's manipulating. Other telekinetics often have similar effects (light blue for Justice/Vance Astrovik, purple for Psylocke, dotted lines or translucent white for the Invisible Woman, etc.). Rarely, if ever, is it made clear whether this energy is supposed to be visible or not. Often, onlookers will clearly be unable to tell why an object is moving seemingly under its own power, but on at least one occasion another character referred to Jean's telekinesis as "pink stuff."
 * For that matter, pretty much every mental power gets a visual representation. Examples include concentric yellow circles for Aquaman's animal control powers, lightning bolts around his head for Professor X's telepathy (rarely used anymore), a wavelike effect for Magneto's magnetic powers, squiggly lines around his head (with half of his face turning into his mask if he's in his civilian ID) for Spider-Man's spider sense, etc.
 * A strange result of this was the case of Ink, a character who thought he had the mutant ability to simulate others' powers by getting an appropriate symbol tattooed on his body (it turned out he was a normal human - it was the tattoo artist who had super powers). One of his tattoo powers was telepathy, which he got from having lightning bolts tattooed on his head, just like the ones used to show Professor X using his powers in old school X-Men comics. The question this raises: how would someone who'd never read an X-Men comic know to associate telepathy with lightning bolts?
 * Doctor Strange's astral self is represented by a 'ghosted' version of himself with various other visual effects. Whether other characters can see him is indicated by dialogue. He also has visual indications when spellcasting, usually in the form of a glow around his hands.

Fan Fiction

 * My Immortal is blissfully unaware of the Rule of Perception, as a result of which characters often seem to materialise out of nowhere.

Film

 * Godzilla versus Megaguirus: We know that large insects are noisy. One would think that Megaguirus, being a monstrous dragonfly-like insect almost as big as Godzilla, would be pretty loud. But no, turns out she is as stealthy as a Ninja. Godzilla and humans alike tend to fail to detect her until she is right on top of them.
 * In Willy Wonka and The Chocolate Factory (circa 1971), none of the visitors recognize that the chocolate river is chocolate until Mr. Wonka tells them. At least one asks something along the lines of, "What is in that river?" Clearly, the chocolate smells no stronger near the chocolate river than anywhere else in the confection-filled room.
 * Epic Movie has a characteristically boneheaded take on the same scenario: Edward drinks from the "chocolate river," and apparently likes it just fine, until someone tells him it's actually a sewage line.
 * Volcano features a scene where a guy jumps off the end of a subway car and is slowly burned up from the bottom, as he sinks into the lava puddle. Only the parts of him in the puddle burn, apparently melting. In reality, lava is very dense, and lava flows can be walked upon. If he'd walked quickly his shoes would have been destroyed, but that should have been it.
 * Or everyone would be roasted alive from being in an enclosed space with a river of molten rock and the extremely hot air surrounding it.
 * In Star Trek VI: The Undiscovered Country, Scotty is shown sitting alone in the conference room studying some blueprints shortly before discovering a major plot point. The blueprints consist of small-scale external views of the Enterprise. This is the sort of basic information we'd expect the chief engineer to have committed to memory. But it tells the viewer that Scotty is hard at work, better than a random electrical schematic might.
 * In the 2010 version of True Grit, the speed of sound issue is noticibly averted. When Rooster Cogburn fires a rifle as a signal from across a valley, we see a plume of smoke shoot silently out of the gun, followed seconds later by the distant crack.
 * Rule of Perception is one of many tropes that The Coen Brothers make a habit of averting and subverting in most of their films.

Literature

 * Thursday Next retreats to a fictional world for a while in the The Well of Lost Plots, and notices several things, like wallpaper, underwear, and breakfast, are missing because they're not usually mentioned in stories. She also finds that she is one of the only people with a sense of smell.
 * And a sense of hearing, at least as we understand it. Although Bookworlders aren't deaf, they can only hear what's explicitly stated in the text. For example, they can't distinguish voices unless something like "Thursday said" appears after the quote.

Live Action TV

 * In an X-Files episode (might have been "The Pine Bluff Variant"), Mulder and Scully's conversation is bugged with a laser beam against the window of Mulder's apartment. The laser is bright red (so we know it's there), instead of infrared, which would be a lot more discreet.
 * The telepathy variant is averted in Heroes: when Matt Parkman reads someone's mind, all the audience usually hears is a jumble of sounds, with an occasional clear sentence bubbling through the chaos.
 * Many of the other powers in Heroes take this to an almost absurd degree. Peter and Ted's hands glow when they are emitting radiation. Elle's electricity is in the form of blue sparks. Sylar's lie-detection skill is indicated with a shake of the camera (and usually his saying "You're lying!" directly aferwards).
 * A Law and Order episode had the detective miss a dead body directly in their line of sight until the camera could see it.
 * In a CSI arson investigation, Greg must compare a used match from the crime scene to a large pile of matchbooks taken from a suspect's home. In through-the-microscope views of him holding the torn match end to the matchbooks, the used match's cardboard shaft is dark in color, while the books' matches are light. This makes it more obvious to viewers that they aren't a match for one another, but begs the question of why Greg bothered comparing those samples microscopically at all, when their colors are so visibly different.
 * Doctor Who just about always has the characters hiding in plain sight of the audience -- where those characters ought to be clear to the searchers, too.

Music

 * In the music industry, if you're not in the public eye people assume you're not doing anything, or have "Fallen Off". Usually happens when an artist isn't properly promoted, or ignored by media outlets.
 * Bone Thugs-n-Harmony hit this trope HARD in the 2000's, They would release mainstream albums and people would still say "I thought they broke up?", Or "I didn't know they had a new album out". Despite having previous albums people seem to have short attention spans thanks to the Fleeting Demographic Rule.

Video Games

 * The Eve's Garden strip club in Bioshock probably qualifies; the sign includes the anachronism "XXX", which would not come into use in Real Life until the 1970s. But players instantly recognize the shorthand.
 * Any Visual Novel with multiple routes, and the challenges facing the heroine in her route are assumed to occur regardless of whether or not the main character is involved with her. Potential Nightmare Fuel without this trope, especially if the heroine will meet a horrible fate without the intervention of the main character. Does not apply if intervention by the main character causes her suffering circumstantially or directly.

Web Comics

 * Derelict: A UV weapon that glows violet.
 * The Cartoon Chronicles of Conroy Cat naturally toys with this one now and again. The "off panel" aspect of this trope is one that Conroy picks up on incredibly quickly during his 'toon training, to Doggy's irritation.
 * In Order of the Stick, whenever Elan plays his bard song, green musical notes fill the air. As part of the strip's general Medium Awareness, one (green) character notices that they are the same color as she is.

Western Animation

 * In the second season pilot of My Little Pony Friendship Is Magic, Discord corrupts Twilight Sparkle's friends one by one. When the ponies turn into jerks, they also become sepia-toned and later turn gray. Spike and Pinkie Pie are the only ones to note the color change, and both characters are among the only ones to have openly broken the fourth wall before.

Real Life

 * In his classic essay "That Which Is Seen, and That Which Is Not Seen", the 19th century French economist Frederic Bastiat argues that many destructive government policies arise from tendency of people to only focus on what is obviously visible. One example he uses is, the savings in tax money that come from discharging troops from the army is diffuse and hard to see, while the difficulty of reintegrating the troops into the workforce is concentrated and easy to see, so people (in Bastiat's era) oppose discharging troops even when the net results would probably be good.
 * Free-trade advocates say the same thing about protectionism nowadays. Raising import tariffs to protect a local factory forces everybody to pay more for that particular good, and therefore more jobs are lost than merely the ones in the factory; they're just less noticeable because they're distributed around all of the companies using that good rather than concentrated in one place.
 * Similarly, the constant public demand for what is known in the UK as "bobbies on the beat" - i.e. a visible police presence. While it might deter some petty street crime, the police often claim that it's an inefficient use of resources (those two officers could be investigating serious crimes full time, rather than standing around on the off chance). The fact that the public has a disproportionate interest in petty street crime in the first place is yet another level of this trope.
 * As one who's studied Criminal Justice, this troper can tell you that while statistically, violent crime rates in the US have declined in the past 20 or so years, every time the news media runs even one kidnapping, rape, or murder story (almost always featuring good-looking, well-off white people as victims), paranoia escalates, and people talk as if the streets are even more dangerous than in the so-called "good old days". Furthermore, while "obvious" (read: blue-collar) crime (murder, rape, assault, arson, burglary, mugging, carjackings, and other felonies the would-be vigilante can imagine himself heroically foiling) get the headlines, less-noticeable white-collar crimes (embezzlement, government corruption, fraud, corporate negligence) can be far more massively devastating on economies, environments, and entire societies.
 * Observing a quantum phenomenon actually changes the output of the quantum effect.
 * Allegedly. How do you know the output and whether it has changed unless you measure it?
 * Schroedingers Cat strikes again.
 * I'll have my gun ready.
 * More accurately would be to say that the outcome is not determined until observed. Until then, it is simultaneously all outcomes at the same time.
 * It's more confusing than that. It derives partially from the fact that you can't know both the velocity and the position of any particle at the same time. What we know is that until we observe it we can only use probability to guess what is going on, we don't know what exactly happens when we observe it that causes it to become certain. My favorite theory is that we are in the Matrix and the computer doesn't bother to calculate an answer unless one is needed.
 * Part of the reason is that our main way of perceiving things is bouncing photons, or for smaller things, electrons off of them. For things at the quantum level, this can actually have a significant impact on behavior.
 * ^That is a faux reason taught to explain a concept that has no cause but simply arises through the maths of the problem. The sum of the uncertainty in position and the uncertainty in momentum equal h, where h can be no less than h/4pi, so you can know all of one and none of the other or bits of each.