A character thinks and thinks... then, suddenly, has a bright idea! A light bulb turns itself on directly above the character's head. Often with character pointing one index fingers upwards.
This trope has been subverted for decades — it became a cliche almost as soon as it was invented (in the 1920s, for the black and white Felix the Cat cartoons).
Often parodied. Most common subversions are the appearances of a broken light bulb to signify someone is thinking of a bad idea; candles, torches, or other sources of light appearing over the heads of characters who pre-date the lightbulb; and a character getting the bulb, but turning it off, indicating they had second thoughts and decided it wasn't a good idea after all. In shows with a weak fourth wall, characters may even grab the lightbulb off of their head and utilize it in their plan, or may happen to be standing under an actual lightbulb that is turned on by another character. Characters Genre Savvy enough will sometimes put an actual lightbulb above their head or someone else's in an effort to give them an idea. A Dead Horse Trope, definitely. Nearly every example here is a parody or subversion. Often used as part of a Rebus Bubble.
Also, this trope may have been invented in reference to a Mr. Thomas Edison having a great idea about something... what was it again?* Oh yes! The phonograph. The lightbulb? No, he didn't actually invent that one. (Or it could just be the concept of enlightenment. Or having a bright idea.)
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Anime and Manga
In one episode of Rozen Maiden: Traumend, a cell phone signal icon with three bars appears over Suiseiseki's head.
Not quite the same thing, but the first episode of Ouran High School Host Club uses a set of light bulbs turning on to indicate the members of the Host Club realizing Haruhi is really a girl. The first bulb to turn on is Kyouya, and the last is Tamaki. When Tamaki's bulb turns on, it then zooms out to show a whole wall of bulbs lighting up to form the kanji for "female".
Parodied in The World God Only Knows: when Elsee gets an idea, a different image pops up each time - firefly!Elsee, traveller!Elsee sighting a star, Elsee powering a lightbulb with a bicycle, etc.
A little lightblub appears over Asuna's head when she realizes that her anti magic fan (which is useless against all the non magic stuff in this arc) is the perfect weapon for enemies like Takane, who clothes herself with shadow magic and has numerous shadow constructs as her fighting style.
In one episode, Tsuruya gets this when she gets an idea about the leftover chocolate.
This was used yet again in the Halloween episode, in which Haruhi gets this.
A promotional spot for the animated version of My Friend Rabbit showed the author of the original book, Eric Rohmann, with an idea bulb over his head.
Astérix uses a burning torch or an oil lamp, since the series plays nearly two thousand years before the invention of the light bulb.
In the Wuzzles section of a Walt Disney Comics comic book, Croc had one appear in a thought balloon, but Flizzard swiped it out of curiosity. (Croc still had the idea.) Later in the same story, Flizzard got out the bulb and held it up, hoping it would turn on and trigger an idea. It did.
In Larry Gonick's Cartoon History of the Universe, a caveman having a brilliant idea has a light bulb appear above his head. When he reveals the flint knife he's created, another caveman says "Oh, I thought it was going to be a light bulb."
A Don Martin cartoon in MAD had a brain surgeon cut open a patient's skull and find a lightbulb (in a socket attached to a cord) inside the head. When the surgeon turns the light on, the patient makes the Aha! reaction.
De Kiekeboes: In Album 26 the family is locked inside a darkened room and wonders how to get out of this situation. Then Fanny says: I have an idea! And when comic book characters have an idea a light bulb appears. And indeed, soon enough a light starts burning, making them able to see what is happening around them.
Films — Animated
Parodied in Over the Hedge, with ideas appearing as lightning bugs (or fireflies, as some o' you may call 'em) slowing in flight and lighting up over the head of the character getting the idea.
In The Curse of the Were-Rabbit, when Wallace gets the idea to use his brainwashing machine to make the rabbits he has captured hate vegetables, the light on his van turns on...though that was Gromit's doing.
Darla Dimple in Cats Don't Dance actually has her butler hold a lightbulb over her head and say "Ding!" when she has an idea.
Gru from Despicable Me also says "Lightbulb!" when getting an idea.
Films — Live-Action
Played with in Home Alone 2. The concierge (played by Tim Curry) of the hotel where Kevin is staying gets suspicious and runs a check on Kevin's father's credit card; when he finds the card was reported stolen, a light in an alcove over his head suddenly turns on. Although given the devilish smile on Curry's face at this point, the modern, oval-shaped bulb could also (ironically) symbolise a halo, given that the concierge is planning something dastardly.
In Hot Fuzz, when Nicholas Angel gets an idea towards the end of the movie, he says 'Idea!' in a dramatic voice.
Edgar Wright used this trope literally saying 'Idea!' in A Fistful of Fingers with lightbulbs on sticks.
In Bio-Dome, one character simply shouts "Lightbulb!" when he gets an idea.
In Inspector Gadget, when Gadget tries to think hard, a lightbulb rises out of his hat. When he gets an idea ("Go go gadget oil slick!"), it turns on. Subverted in that it was a bad idea to actually say this out loud, thereby spraying his Love Interest with, inexplicably, not oil, but toothpaste.
In Bedtime Stories, Skeeter is fixing a lamp as his nephew talks about the bedtime story they told last night. When he notices that it was only the story elements contributed by the children that come to pass in the real world, the lightbulb comes on.
In Mel Brooks's Silent Movie, Mel Brooks plays a movie producer who's down on his luck. He's produced only flops in recent years, and the evil corporation "Engulf and Devour" is about to shut his little studio down. He sits down dejectedly in a chair against the wall, trying to think of some way to save his studio. Suddenly, he gets an "AHA!" look on his face ... and the light fixture on the wall directly over his head spontaneously lights up.
Less of an idea than it is a revelation, but when Cher in Clueless finally realises that she's in love with Josh, an entire fountain lights up behind her. Granted, it's not a lightbulb, but it's still the same basic idea of external light = inner illumination. (It's also a fountain, which has its own innuendo.)
It even shows up in The Big Sleep, as Bogart is sitting on a couch with a lamp behind him, and someone turns it on just after making an off-hand comment that brings half the plot into focus (for Marlowe, anyway, if not the viewer).
Spoofed in Adele Hasn't Had Her Dinner Yet (1977, Czechoslovakian movie). Nick Carter is the Greatest America's Detective and he's friends with Thomas A. Edison. There is a signed photo with Edison with a picture of a bulb. The message in the photo reads: My dear Nick, thank you for your advice. It was a million dollar idea. Gratefully yours, Thomas A. Edison
The Australian children's stories about Bottersnikes And Gumbles feature a variant there's no bulb, just the "tink" sound that usually accompanies it. It's limited to a particular character, and the loss of his "tink" is a plot point in one book.
Literal example in Robert Rankin's The Book Of Ultimate Truths. We all have cartoon style thought bubbles (or sleep Zzzzs) all the time. One certain monk has a terrible affliction whereby he can see them, and we can see his. We know that a character has an idea when the monk observes the flashing light bulb.
Saved by the Bell: Zack hears that tomorrow is a Jewish holiday; a menorah appears over his head, and he decides to get excused so he can see a baseball game. (Yes, it's in live-action too.)
One fairly unique live-action example involved no post-production effects at all: the set of the short-lived UK TV series Mr Don and Mr George had several unobtrusive lights on various points on the walls. Through careful positioning of the cameras and actors, whenever a character had an idea, a light would light up behind them, appearing to be just above their heads. This was done so smoothly and effectively that even though the gag was done several times, the audience never saw it coming.
This exact same thing was done in Detroit Rock City, when the main characters are trying to come up with a way to enable one of their number to get out of tech class to call a radio station for KISS tickets.
Frasier: When the two brothers are writing a book, Frasier is struck by inspiration (nothing appears):
Frasier: Niles, do you see a light bulb over my head?
Used repeatedly in one episode of That '70s Show. First during a school assembly regarding the President's visit to town, where the various teenagers have actual lightbulbs turn on when one aspect of the speech caught their interest (for instance, Hyde when the principal mentions that they shouldn't cause trouble to make a statement against the government), culminating when the principal says that they don't like it, they can just leave (at which point the "Exit" sign above Fez's head lights up and he walks out the door).
It came back later that same episode when Red got an idea and the porch light behind him turned on. Played with when he turns around and tells Eric to turn the light off.
Hannah Montana plays with the trope when Mamaw buys her a goofy hat with lightbulbs in it for an awards show. When she hatches up a Zany Scheme in mid-conversation in the wardrobe closet to convince Mamaw and Aunt Dolly to get along, the hat lights up, spooking out Lilly.
In Dinosaurs when Charlene comes up with the idea that the world is round a light bulb appears above her head, which is actually just Earl fixing the lamp in the background.
Kamen Rider Fourze parodies this. The eponymous hero, Gentaro, has a Eureka Moment during a special test, and as a visual, an Astro Switch was shown when the realization hits. Instead of the Flash Switch as an appropriate Visual Pun, we instead see the Hand Switch. Probably has something to do with Gentaro being an Idiot Hero...
In an episode of Parks and Recreation in which Ben gets a job at a new office, after a particularly clever idea a colleague gets up, stands by the desk lamp behind Ben, and gets him to re-enact the moment he thought of it so his colleague can switch on the light at the right moment.
In the Supernatural episode "What Is And What Should Never Be", Dean sees the light bulb above his head in the warehouse, and he has a revelation. There is an electrical whine, and the light flickers before Dean sees himself bound and being drained of blood and realizes he is a victim of the djinn.
Parodied in FoxTrot when, with Paige alternately struggling with her report on Thomas Edison and her father's attempts to help her, a lightbulb appears just over her head - held by her dad, who begins to reel off Edison's inventions. (The look on her face sells it.)
Or another where Jason makes a mobile out of lightbulbs.
In yet another Paige tries to invoke this trope by taking the bulb out of her lamp and holding it over her head; Peter says that he only does that with 100-watt bulbs.
Parodied in Pearls Before Swine, when Pig comments that they don't have an extra lightbulb to replace a dead one, Rat says that he has an idea. He then uses the bulb that appears above his head as a replacement. His idea was to have an idea?
Sydney has an idea depicted (and labelled) as a "PC compact fluorescent bulb."
A fluorescent energy saving lamp appears above one of the protagonists' head. This is of course lampshaded in a caption that also points out that this is the politically correct way to signal an idea.
In Drabble, Ralph gets an idea, complete with bulb. His wife glares at him, and in the next panel, the bulb is a more energy efficient compact fluorescent one. He looks annoyed, and his wife is now smiling.
In The Wizard of Id, the title character is, in the first panel, straining hard to come up with an idea. Panel two, his face brightens, and a light bulb appears over his head. Panel three, the scene is drawn at a wider angle to show a huge pile of light bulbs next to him. Wizard grumbles, "I need ideas. I get light bulbs."
Played with twice in Wallace & Gromit's Grand Adventures: The Last Resort. As Wallace gets an idea in the first act, a few sparks in the plug behind him go off above his head. In the second act, it's a sun lamp.
In the Updated Re-release of Tales of Graces, Tales of Graces f, Pascal gets one in Lineage and Legacies in the Eleth Research Laboratory following the boss fight when the party realizes something is wrong with Fodra's core and she's asked if she has any ideas.
Lightning Returns: Final Fantasy XIII: There is a side quest to find "adornments" for Candice the baton-twirler. When you give her some, a bulb lights up above her head as she has an idea about how to wear them.
And in, this one Piffany, being a cleric, has a burning bush.
Brawl in the Family: In the middle of this comic, Table Etiquette, Kirby and Jigglypuff were thinking of another way to finish their food more quicker, until Jigglypuff gets an idea, and she has an Idea Bulb appear above her head.
Meanwhile, in this part of Cocoon Academy, Mario gets an Idea Bulb.
Played with in The Inexplicable Adventures of Bob! Bob is trapped by villains in a small room, and thinks about how he desperately needs an idea. Then he glances up and sees there's an actual mounted lightbulb directly above his head. He smashes it and escapes in the darkness.
In this comic, Larisa, instead of a light bulb, gets a bonfire when she notices the principal's report. Then she proceeds to use the bonfire to set the pricipal's report on fire, before dropping it in the trash bin, which sets off the fire alarm and the rain that appears puts out the bonfire she used.
And to think that using a "devilishly idea bulb" in this comic can't push it to the limit.
Betty Boop: In 1935, Fleischer Studios introduced a new character, Grampy, in Betty Boop and Grampy (available for download at archive.org). In this and subsequent episodes, when Grampy is called upon to solve a problem for Betty, he puts on his mortarboard Thinking Cap with light bulb mounted on top. The bulb lights up when he has the solution.
In an episode of The Fairly OddParents, things go back to Colonial Times, and candles are used in lieu of bulbs. So, Timmy gives people ideas by holding candles over their heads. Wax often falls on the characters' heads, after which they say something along the lines of "I have an idea! ...and wax burns!"
Similarly, early on in the low-tech Avatar: The Last Airbender, when Sokka comes up with an idea (one his first instances of later becoming the The Smart Guy) for opening a sophisticated chamber door, he has his back to a wall and there is a lantern above his head.
Sometimes appeared in with the bulb replaced by matches being struck; this was eventually dropped along with Beavis' pyromania to appease Media Watchdogs.
Whenever this appeared in later episodes, it was usually depicted as a small, noisy, flickering bulb (giving the viewers an idea of the soundness of the idea); it apparently had to shatter before the idea could be verbalized by the character who had come up with it.
In Darkwing Duck, the villain Megavolt has a literal lightbulb attached to his head, as an indication of his thought processes.
A light bulb appears over Yakko's head at least once. Wakko promptly ate it, which hilariously resulted in Yakko forgetting the idea he just had.
The "Good Idea, Bad Idea" segments.
In The Pink Panther short, "Psychedelic Pink", the Pink Panther (the cartoon character, not the gem) is trying to read "The Love Life of a Panther (uncensored)". He finds a room with a chair and a lamp. As with most light bulbs he encounters when he's trying to do something, however, it has a habit of turning off when it's least convenient to turn it on again. He sees a vending machine for lights, and having the wrong kind dispenses three times, he gets the idea to use the bulb resulting from the idea as a regular light bulb. It is unknown whether or not it would have worked, though, because it gets a quarter-second of operation before the lampshade falls down, causing the rest of the lamp to disappear.
In an episode of Duck Dodgers, a lightbulb appears over Dodgers' head when he gets an idea. So he stops explaining his idea to complain to the Cadet that the hologram projector is acting up again.
In the episode "Rock TV" of Super Mario World, Bowser had three lit candles that played the NBC chimes.
Histeria! played with this in the sketch about the inventor of the lightbulb, Thomas Edison. Throughout the sketch, he tries to think of a good invention and a light bulb appears above his head with each thought. After quite a while, Loud Kiddington points it out to him, and he decides to call it "the thing that clicks on over my head when I get an idea." Toast tells him to "lighten up" and call it a light bulb.
Secret Squirrel: The supervillain turned off every single light in the world and covered it in absolute darkness. Then Squirrel thinks of an idea to get a bulb on his head so he can have some light.
In The Beatles cartoon "You Really Got a Hold on Me", the four are lost in a jungle with a jeep that stopped working. After applying thought Ringo gets an idea with the cliche bulb over the head image. Catch here is a huge snake appears and swallows the bulb! Then on the reptile shines a beacon every time it opens its mouth!
Back to the FutureThe Animated Series: Doc gets an idea of how to test his new invention. A lightbulb is behind him - but then Doc walks away, revealing he is standing in front of a lamp, which Verne then switches off.
PB&J Otter! Whenever a character, usually Jelly, got an idea at the end of a Noodle Dance, a lightbulb would appear. Curiously, though, it would generally appear off to the left, rather than directly above the character's head.
In the animated series of Beetlejuice, Beetlejuice gets one, but has no idea why it's there. He looks up at it and wonders if it means he's gotten an idea.
In Rocko's Modern Life, Heffer gets an idea, but no bulb appears. He quickly asks Rocko what's the "idea symbol." When Rocko answers that it's a lightbulb appearing over your head, Heffer actually makes one pop out the top of his head.
In one Disney cartoon, Chip 'n Dale are trying to think of a way to get even with Donald Duck. Dale has and idea, a light bulb appears over his head, and he shows it to Chip, who rejects it. Trying again, Dale comes up with a bigger bulb, which Chip approves.
In the Will And Dewitt story "You Bet," the frog Dewitt gets an idea, but an Idea Bulb appears over Will's head. Dewitt gripes, "Hey, I'm the one who got an idea," swipes the bulb and places it over his head. He then notes "Hey, we're trying to save energy" and the standard-style bulb changes into a CFL.
On Dexter's Laboratory it was parodied with Dexter and Dee Dee getting simultaneous ideas. Dexter's was represented by two atoms undergoing fusion and producing light. Dee Dee's was represented by two rocks banging together and making fire. Of course, since the previous is a natural process, while the latter represents one of the first ideas from which all science descends...guess which plan turned out better?
A student film called "Employee Of The Month" entirely centers around this conceit. The Boss is looking for big ideas — there's even a chart for comparison, although most of the protagonist's ideas don't match up (not even the one he uses to steal his rival's idea, which is bigger than he is). The Stinger reveals that The Boss is a giant moth in a business suit, who wants big ideas because he likes the pretty glowy things.
Fanboy and Chum Chum: In one episode, Fanboy gets an idea bulb, but it burns out. However, Chumchum fixes it by walking up invisible steps with a new lightbulb, and replaces the burned out one with the new lightbulb.
In Dan Vs. "The Salvation Armed Forces'', Dan demands Chris come up with an idea. As Chris gazes across the street, a truck for Idea, Inc. drives by, complete with a smiling lightbulb mascot painted on the side.
Used in an episode of Ruby Gloom when Iris and Misery try to think of ways to get Skullboy's attention (Love spell, long story) a couple of idea candles appear above their heads.
In the episode "Family Appreciation Day", Scootaloo excitedly jumps into the air when she gets an idea of how to get out of Apple Bloom bringing Granny Smith to Family Appreciation Day, and she just happens to jostle a lantern hanging over her and turn it on.
In "One Bad Apple", we find the spot under the lamp is often where the CMCs go to think.
In the Mike, Lu & Og episode "Sparks", Og gets the idea to invent the lightbulb. In this case, his "bulb" is represented by a thought bubble containing a candlestick.
In the CatDog episode "Dogpower", the titular duo are unable to pay their electricity bill and thus are forced to go without power for awhile. When their housemate Winslow notes Dog being full of energy whenever he enthusiastically chases things, an inspirational light bulb springs up above Cat's head. However, it promptly goes out, forcing him to flick it with his finger to keep it on, suggesting that their present situation means even visual metaphors are low on power.
SpongeBob SquarePants, episode "SpongeGuard on Duty": Spongebob gets a bright idea and momentarily assumes the shape of a light bulb.
Hildy from The 7D will often have a candle appear over her head when she gets an idea.
Ultimate Spider-Man has one appearing next to Spidey as he hatches an idea on how to stop Venom with a symbiote sample. He changes it from an incandescent bulb to a CFL bulb as it was "better for the environment".
Oddly enough, the human brain uses about 100 Watts — enough to light one of the largest standard lightbulbs (or several compact florescents). "Luminous beings are we" indeed.
Then again, if the brainpower was being used to power a lightbulb there wouldn't be enough left to power the brain...