Engineering Magic Tricks: The Science & Effects Behind Illusions
Illusioneering is the process of creating magic tricks, often through exploiting strange phenomena discovered in the STEM (science, technology and engineering) subjects as brain hacks. Trick interrupt the human senses of causality and continuity; they disrupt our perception of cause and effect. Strange quirks of physics are indistinguishable from magic.
Designing magic tricks is a lot of fun. You might not get the glory, but you can certainly watch by the side of the stage with a huge smile.
Pepper's Ghost: The Original
Magicians often use mirrors, lenses, and other optical devices to create illusions. For instance, the classic "Pepper's Ghost" illusion uses a combination of mirrors and light refraction. It has been used in various forms throughout history, from Victorian theater performances to modern attractions like the Haunted Mansion at Disneyland and even holographic performances featuring deceased musicians.
Pepper's Ghost is an illusion technique that dates back to the 16th century but gained popularity in the 19th century after English scientist John Henry Pepper adapted the effect for the stage. The technique creates the appearance of a ghostly figure or object seemingly floating in mid-air. It has been widely used in theaters, amusement parks, and even television and film.
The illusion works by using a large, transparent piece of glass or plastic, called a "Pepper's Ghost foil," which is placed at an angle between the audience and the main stage. The "ghost" is an object or actor positioned in a concealed area, often referred to as the "blue room," off to the side of the stage. The lighting in the blue room is controlled so that the audience cannot see the hidden area directly.
When the light in the blue room is turned on, the image of the ghost is reflected onto the angled glass, creating a transparent, ethereal appearance. The audience sees the reflection, which appears to be interacting with the real objects or actors on the main stage. By adjusting the lighting in the blue room and on the main stage, the illusion can be made to appear or disappear, giving the impression of the ghostly figure materializing or vanishing.
Exploiting Scientific Phenomena
Science is a beauty menagerie of strange and eerie mysticism if you look close enough; all of which is easily explained, but easily fools the mind and senses.
The classic example is flight: airplanes fly by a magical principle of physics. Fast-flowing air is at lower pressure than slow-moving air. The pressure above an airfoil is lower than the pressure below. It results in lift; a force that powers the plane upward due to Bernoulli’s law.
But there are many others:
- Refraction: Light bending through materials can create illusions of invisibility or distortion.
- Magnets: Magnetic forces can create illusions of levitation, attraction, or repulsion.
- Surface Tension: The cohesive force of liquids can create illusions of floating objects or liquid manipulation.
- Inertia: An object's resistance to change in motion can be used to create illusions of balance or stability.
- Gyroscopic Stability: Spinning objects can create illusions of defying gravity or maintaining balance.
- Air Pressure: Differences in air pressure can create illusions of levitation or implosion.
- Static Electricity: Electric charge buildup can create illusions of attraction, repulsion, or hair-raising effects.
- Capillary Action: The movement of liquids through narrow spaces can create illusions of liquid defying gravity or self-filling containers.
- Chemical Reactions: Color changes, exothermic reactions, and gas production can create illusions of transformation, heat, or smoke.
- Phosphorescence: Glowing materials can create illusions of light without an apparent source.
- Mirage: Refraction of light through layers of air can create illusions of objects appearing or disappearing.
- Buoyancy: The upward force of fluid can create illusions of floating or suspension.
- Elasticity: The ability of materials to return to their original shape can create illusions of self-healing or shape-shifting objects.
- Ferrofluids: Magnetic liquids can create illusions of fluid manipulation or shape-changing materials.
- Thermochromism: Temperature-sensitive materials can create illusions of color change or revealing hidden messages.
- Acoustic Levitation: Sound waves can create illusions of objects levitating in mid-air.
- Piezoelectricity: Mechanical pressure generating an electric charge can create illusions of electricity control or energy transfer.
- Perceptual Adaptation: The brain's ability to adjust to altered sensory input can create illusions of size or shape distortion.
- Binaural Beats: The perception of a third tone when two tones of slightly different frequencies are played can create illusions of auditory hallucinations or mood manipulation.
- Hydrophobic Materials: Water-repellent materials can create illusions of liquid manipulation or resistance to staining.
- Supercooling: The cooling of a liquid below its freezing point without it becoming solid can create illusions of instant freezing or ice manipulation.
Understanding the 19 Types of Magic Tricks
"The Trick Brain" is an infamous 1944 book by magician Dariel Fitzkee that categorizes magic tricks into 19 fundamental types or effects.
- Production: The magician creates or produces something from nothing, such as making an object appear out of thin air.
- Vanish: The magician makes something disappear, like making a coin vanish from their hand.
- Transposition: Two or more objects change places; for example, a signed card appearing in a spectator's pocket while the card in the magician's hand is revealed to be a different one.
- Transformation: An object changes its properties, like a silk handkerchief changing color.
- Penetration: The magician causes an object to pass through another object, such as a coin passing through a solid table.
- Restoration: The magician seemingly destroys an object, then restores it to its original state, like cutting a rope and then making it whole again.
- Animation: The magician brings an inanimate object to life or controls its movement, like making a handkerchief dance.
- Anti-Gravity: The magician defies the laws of gravity, like floating a playing card in mid-air.
- Attraction: The magician demonstrates a seemingly supernatural control over objects or forces, like bending a spoon with their mind.
- Sympathetic Reaction: The magician influences an object's behavior based on the behavior of another object, such as causing two rings to link together after tapping them against each other.
- Invulnerability: The magician demonstrates immunity to harm, like pushing a needle through their hand without injury.
- Physical Anomaly: The magician exhibits extraordinary physical abilities, like swallowing a sword or walking on fire.
- Spectator Failure: The magician creates a situation where the audience member cannot accomplish a simple task, like lifting a small object.
- Control: The magician demonstrates control over another person or object, such as predicting a chosen card or controlling someone's actions.
- Identification: The magician identifies a chosen object or person among a group, like finding a specific playing card in a deck.
- Thought Reading: The magician appears to read the thoughts of the audience, like revealing a spectator's secret word.
- Thought Transmission: The magician communicates their thoughts to a spectator, like transmitting a number mentally.
- Prediction: The magician predicts the outcome of an event, like correctly guessing the roll of a pair of dice.
- Extra-Sensory Perception: The magician demonstrates the ability to perceive things beyond the normal senses, like identifying an object in a closed container without looking.
How do you make a grand piano disappear? David Berglas explains!
"13 Steps to Mentalism" is a 1961 book written by British magician Tony Corinda, which serves as a comprehensive guide for learning and mastering mentalism - tricks which duplicate "supernatural" powers and "psychic" performances. The book is divided into 13 chapters or "steps," each focusing on a specific aspect of mentalism.
- Swami Gimmick: a secret writing device that enables the mentalist to create the illusion of predicting future events or reading someone's thoughts.
- Pencil, Lip, Sound, Touch, and Muscle Reading: techniques to seemingly read a spectator's mind by observing subtle cues, such as pencil movements, lip reading, listening to sounds, feeling muscle tension, and touch.
- Mnemonics and Mental Systems: memory techniques and mental systems to enhance the mentalist's abilities, including memorizing decks of cards, names, numbers, and other information.
- Predictions: methods to create convincing predictions, such as sealed envelopes, newspaper predictions, and other techniques that create the illusion of foretelling future events.
- Blindfolds and X-Ray Eyes: methods for performing blindfolded feats and convincing audiences of the ability to see through solid objects or read hidden information while blindfolded.
- Billets: small pieces of paper (billet envelopers) to secretly obtain, exchange, or reveal information, creating the illusion of telepathy, clairvoyance, or other psychic abilities.
- Book Tests: methods for performing mind-reading effects using books, such as revealing a word or passage chosen by a spectator.
- Two-Person Telepathy: techniques for creating the illusion of telepathic communication between two performers, often involving secret codes or signals.
- Mediumistic Stunts: methods for simulating mediumistic or spiritualistic phenomena, such as communicating with spirits, spirit writing, and table tipping.
- Card Tricks: card tricks specifically designed for mentalists, emphasizing mind-reading and prediction effects rather than traditional sleight-of-hand.
- Question and Answer: techniques for performing a "Q&A" act, in which the mentalist appears to answer questions from the audience using psychic or telepathic abilities.
- Publicity Stunts: ideas and methods for gaining publicity as a mentalist, such as performing psychic feats for the press, predicting headlines, and other attention-grabbing stunts.
- Patter and Presentation: the importance of presentation, scripting, and audience engagement in mentalism, offering tips for developing a strong stage presence and delivering convincing performances.
Example: The Science Of Levitation Effects
How do you make something appear as if it is floating in the air, against gravity? Well, there are several ways of achieving it, and all of them are feats of intellect rather than supernatural forces.
Balancing and counterweighting
A hidden support or counterweight system is used to balance the person or object being levitated, giving the appearance that they are floating in mid-air. The support is carefully concealed from the audience's view.
Strings and wires
Invisible strings or thin wires can be employed to suspend objects or people in the air. The wires are typically made from a material that is difficult to see under stage lighting or against a particular background, making the levitation seem genuine.
To support a 200lb (90.7 kg) man using thin stage wires, you need to ensure that the wires can withstand the tension created by the man's weight.
- Calculate the required tension force in the wires: The tension force (F_tension) in each wire should be equal to or greater than the man's weight divided by the number of wires used (n). F_tension ≥ (Weight) / n
- Determine the wire material and thickness: The wire material and thickness (diameter) must be chosen so that the wire can withstand the required tension force without breaking. This depends on the wire material's tensile strength (σ) and the cross-sectional area (A) of the wire.
Tensile strength can be calculated as: σ = F_tension / A
The cross-sectional area of the wire (A) is related to its diameter (d) as follows: A = π (d/2)²
For simplicity, let's assume we are using four wires to distribute the weight evenly.
- Select a wire material: A common material used for stage wires is steel, specifically, galvanized steel aircraft cable, which has a tensile strength of about 1,700 MPa (megaPascals) or 1,700 N/mm².
- Determine the required wire diameter: Using the tensile strength formula and assuming a safety factor (SF) of 5 for added security, we can calculate the required diameter (d).
σ = (F_tension × SF) / A 1,700 N/mm² = (222.475 N × 5) / (π × (d/2)²)
Solving for d, we find that d ≈ 1.9 mm.
So, a suitable wire material would be galvanized steel aircraft cable with a diameter of approximately 1.9 mm, using four wires to distribute the weight.
Using powerful magnets or electromagnets, objects can be made to appear as if they are levitating. The magnetic force is strong enough to counteract gravity, causing the object to hover in place. This principle has been used in some levitating train and toy designs.
To levitate a 200lb (90.7kg) man using magnets:
- Calculate the gravitational force (weight) acting on the man: Weight = mass x acceleration due to gravity where mass = 90.7 kg, and acceleration due to gravity (g) ≈ 9.81 m/s². Weight = 90.7 kg × 9.81 m/s² ≈ 889.9 N (Newtons)
- Determine the magnetic force required: To levitate the man, the magnetic force (F_magnetic) must be equal to or greater than the gravitational force (F_gravity). F_magnetic ≥ F_gravity F_magnetic ≥ 889.9 N
- Design the magnetic system: Creating a magnetic levitation system for a human requires strong magnets or electromagnets, as well as a stable configuration to prevent the person from flipping over or falling due to instability. One possible method is using diamagnetic materials (e.g., superconductors) or a combination of permanent magnets and electromagnets arranged in a stable configuration, like the Earnshaw's theorem (https://en.wikipedia.org/wiki/Earnshaw's_theorem)-stabilized magnetic levitation system.
- Calculate the magnetic field strength and gradient: The actual magnetic field strength (B) and magnetic field gradient (∇B) required depend on the type and arrangement of magnets used. The calculations are more complex and require detailed knowledge of the specific magnetic materials and the geometry of the magnetic system.
In certain cases, air pressure can be harnessed to create levitation illusions. By directing a strong airflow towards an object, the force of the air can be used to counteract gravity and cause the object to float.
To levitate a 200lb (90.7 kg) man using air pressure, you need to generate enough upward force to counteract his weight.
- Calculate the gravitational force (weight) acting on the man: Weight = mass × acceleration due to gravity where mass = 90.7 kg, and acceleration due to gravity (g) ≈ 9.81 m/s². Weight = 90.7 kg × 9.81 m/s² ≈ 889.9 N (Newtons)
- Determine the required air pressure: To levitate the man, the air pressure force (F_air) must be equal to or greater than the gravitational force (F_gravity). F_air ≥ F_gravity F_air ≥ 889.9 N
Air pressure force can be calculated as: F_air = P × A where P is the pressure difference and A is the contact area between the man and the air.
To find the required pressure difference, we need to know the contact area (A). The contact area depends on the method used to create the air pressure, such as an air cushion or platform. Once the contact area is known, we can calculate the pressure difference (P).
P = F_air / A
This technique uses sound waves to levitate small objects. By generating a standing wave with a precise frequency and intensity, the pressure created by the sound can counteract gravity and cause the object to hover in place. While this technique has limited applications in magic performances, it has been demonstrated in laboratory settings.
Acoustic levitation relies on creating a standing wave that generates a pressure node capable of counteracting the force of gravity on the object.
For small objects, the required acoustic pressure (P_acoustic) can be calculated as: P_acoustic = (2 × π × f × ρ × v) / λ where f is the frequency of the sound wave, ρ is the density of the medium (air), v is the speed of sound in the medium, and λ is the wavelength of the sound wave.
Optical illusions and misdirection
Some levitation illusions rely on optical tricks or misdirection to give the appearance of levitation. For example, a person might appear to be floating due to a hidden support structure, while the magician directs the audience's attention elsewhere.
Mirrors and reflections
Mirrors can be cleverly placed to hide support structures or create the illusion of an object levitating by reflecting its image onto a transparent surface. This technique is similar to Pepper's Ghost, where an angled reflective surface creates a ghostly appearance.
This theatrical technique involves using black backgrounds, lighting, and clothing to conceal support mechanisms, such as wires or platforms, that create the illusion of levitation. When done correctly, the audience cannot see the hidden supports, making it appear as if the object or person is genuinely levitating.
Effects & Phenomena: A-Z
There are endless amounts of mechanics found in nature we can use in building illusions, so this is not definitive. But it's a good start. And how you use them is up to your own imagination.
A distorted room that creates an optical illusion making one person appear much larger or smaller than another when they are actually the same size. It exploits the brain's assumption that the room has a normal rectangular shape.
The tendency to rely heavily on the first piece of information encountered when making decisions. For example, an individual may be more likely to purchase a car if it is placed alongside a more expensive model (the anchor).
A stationary light appears to move in the dark due to small, involuntary eye movements. The opposite effect of autokinesis is autostasis: a moving bright light in a dark sky appears stationary.
The tendency to judge the likelihood of an event based on the ease with which relevant examples come to mind. If something can be recalled, it must be important, or at least more important than alternative solutions not as readily recalled.
Baader-Meinhof Phenomenon (Frequency Bias)
The tendency to notice and pay attention to something after learning about it for the first time; to notice something more often after noticing it for the first time, leading to the belief that it has an increased frequency of occurrence.
Barnum Effect (Forer Effect)
A cognitive bias in which individuals perceive vague, general statements as being specifically accurate for them. The basis of astrology/horoscopes and "psychic readings".
- You have a great need for other people to like and admire you.
- You have a great deal of unused capacity, which you have not turned to your advantage.
- Disciplined and self-controlled outside, you tend to be worrisome and insecure inside.
- You prefer a certain amount of change and variety and become dissatisfied when hemmed in by restrictions and limitations.
- You pride yourself as an independent thinker and do not accept others statements without satisfactory proof.
- You have a tendency to be critical of yourself.
- At times you have serious doubts as to whether you have made the right decision or done the right thing.
- At times you are extroverted, sociable, while at other times you are introverted, wary, reserved.
- While you have some personality weaknesses, you are generally able to compensate for them.
Forer, B. R. (1949). The fallacy of personal validation: A classroom demonstration of gullibility. Journal of Abnormal and Social Psychology, 44, 118-123 : https://sci-hub.se/10.1037/h0059240
A color appears to change when surrounded by different colors, demonstrating the brain's relative perception of color. Used to create Munker–White's illusion.
Breathing Square Illusion (Pac Man)
A static square appears to be expanding and contracting due to the motion of the background.
The tendency for individuals to be less likely to help someone in need when others are present. First proposed in 1964 after the murder of Kitty Genovese, a bartender in New York.
Café Wall (Kindergarten) Illusion
Alternating light and dark tiles, which create the impression of a wedge-shaped distortion or bending in the lines of tiles.
The change in frequency of a wave relative to an observer can create illusions of changing pitch or sound manipulation. For example, the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
The tendency for people with low ability in a domain to overestimate their ability, and for people with high ability to underestimate their ability. AKA "stupid people don't know they're stupid."
The tendency to value objects more highly simply because one owns them. People are more likely to retain an object they own than acquire that same object when they do not own it.
A static image appears to be in motion due to the arrangement and interaction of shapes and colors, caused by rapid, unconscious eye movements known as microsaccades.
Flashed Face Distortion Effect
Normal faces appear grotesque and distorted when presented rapidly and aligned at the eyes.
This principle is used to create illusions of size and distance by strategically positioning objects and using angles to manipulate the viewer's perception. Object appear farther away, closer, larger or smaller than they actually are. It manipulates human visual perception through the use of scaled objects and the correlation between them and the vantage point of the spectator or camera.
People fail to notice significant visual changes in their environment. A perceptual phenomenon that occurs when a change in a visual stimulus is introduced and the observer does not notice it.
The perceived contrast of an object depending on the contrast of its surroundings. An object of low-contrast visual texture surrounded by a field of uniform visual texture appears to have higher contrast than when presented on a field of high-contrast texture.
Confirmation (My side) Bias
The tendency to search for, interpret, and remember information that confirms one's preconceptions. Responsible for attitude polarization, belief perseverance, the irrational primacy effect, and illusory correlation.
Ebbinghaus Illusion (Titchener Circles)
Two circles of the same size, each surrounded by other circles: one appears larger than the other because of the size of the surrounding circles.
Fading Dot Illusion (Troxler's Fading)
When one fixates a particular point, after about 20 seconds or so, a stimulus away from the fixation point (in peripheral vision) will fade away and disappear.
Flicker Fusion Threshold
The frequency at which intermittent light stimulation appears to be completely steady to the average human observer. The Critical Flicker Fusion Threshold (TFF) is an index of cerebral nervous system (CNS) function used to measure alertness and cortical arousal.
Gambler's (Monte Carlo) Fallacy
The tendency to believe that the probability of a random event changes based on the outcomes of previous events. The incorrect belief that, if a particular event occurs more frequently than normal during the past, it is less likely to happen in the future (or vice versa).
A moving object appears to be ahead of a flashing object, even when they are aligned, due to the brain's processing of motion. The flash and a moving object that appear in the same location are perceived to be displaced from one another.
The tendency for people to attribute positive qualities to someone they find attractive or likable. Evaluators tend to be influenced by their previous judgments of performance or personality.
Two parallel lines appear to bow outward due to the presence of radiating lines in the background. AKA the "Flying into the Death Star" effect in Star Wars.
A concave facial mask that appears to be convex, demonstrating the brain's expectation of a face to be convex.
An individual makes unconscious, involuntary movements in response to an idea or mental image. Actions too small for you to realise you're making them: derived from the terms "ideo" (idea, or mental representation) and "motor" (muscular action).
This is the principle behind "Ouija" boards and pendulum forecasting.
Illusion of Control
The tendency to believe that one has control over events that are actually determined by chance, and for people to overestimate their ability to control them.
Illusion of Transparency
The tendency for people to overestimate the extent to which their thoughts, feelings, and intentions are known by others. Related to asymmetric insight, whereby people perceive their knowledge of others to surpass other people's knowledge of them.
Illusory Truth Effect
The tendency for people to believe information to be true after repeated exposure to it, even if it is false. Linked to hindsight bias, in which the recollection of confidence is skewed after the truth has been received. AKA "a lie repreated enough becomes the truth."
The tendency for people to place a disproportionately higher value on objects they have assembled or created themselves.
Similar to change blindness, inattentional blindness occurs when people fail to perceive an object or event that is in plain sight because their attention is focused elsewhere. When it becomes impossible to attend to all the stimuli in a given situation, a temporary "blindness" effect can occur.
The belief that the world is fair, and people get what they deserve. Cosmic justice, destiny, divine providence, desert, stability, order, or karma.
The brain perceives a triangle that doesn't exist. The triangle is implied by the arrangement of three Pac-Man-like shapes and three angled lines.
Law of Small Numbers (Hasty generalization)
The tendency to draw conclusions from small sample sizes. Basing broad conclusions regarding a statistical survey from a small sample group that fails to sufficiently represent an entire population.
Liquid droplets on hot surfaces can create illusions of levitation or walking on hot surfaces without injury. A liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this repulsive force, a droplet hovers over the surface, rather than making physical contact with it.
Lilac Chaser (Pac Man Illusion)
A series of disappearing and reappearing lilac-colored dots, creating the perception of a green dot moving around a circle.
The brain perceives a third sound based on conflicting information from the eyes and ears. The visual information a person gets from seeing a person speak changes the way they hear the sound.
The tendency for people to develop a preference for things they are familiar with. The more often people see a person, the more pleasing and likeable they find that person.
The Moon appears larger near the horizon than when it is higher in the sky. The exact cause of the Moon Illusion is still debated, but it is likely due to the brain's misinterpretation of size and distance cues.
Two lines with arrow-like ends: the lines are of equal length, but one appears longer than the other due to the orientation of the arrow-like ends.
Identically colored objects appear as different shades depending on the background and the colors of the surrounding objects.
The tendency to perceive objects as having stable properties, such as size, shape, and color, despite changes in the retinal image; the perception of an object or quality as constant even though our sensation of the object changes.
Persistence of Vision
The human eye retains an image for a fraction of a second after the image has disappeared; when visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. Why a glowing coal or burning stick leaves a fiery trail after it is whirled around in the dark.
Continuous motion between two or more sequentially displayed still images or objects; apparent motion that is observed if two nearby optical stimuli are presented in alternation with a relatively high frequency.
A static image appears to move when the viewer's eyes follow a particular path around the image.
The tendency for people to experience real improvement in their condition due to the belief that they are receiving a treatment, even if the treatment is inactive or fake. People believing their pills worked, when they were actually duds.
A diagonal line passing behind an occluding shape appears to be misaligned when it emerges on the other side.
Two objects of equal size appear to be different sizes because of the context in which they are displayed. Converging lines create a sense of depth and perspective, causing the brain to perceive the objects as different sizes. The "oncoming railway track" idea.
The tendency to remember the first items in a list or sequence more easily than those in the middle.
Subtly influencing a person's thoughts and perceptions by exposing them to related stimuli. If you are presented with the word ‘doctor’. A moment later, you will recognize the word ‘nurse’ much faster than the word ‘cat’ because the two medical workers are closely associated in your mind.
The sense of the relative position of one's own body parts and the strength of effort being employed in movement; the sense of self-movement, force, and body position. Mediated by proprioceptors, mechanosensory neurons located within muscles, tendons, and joints.
AKA "Sixth Sense".
A moving object is perceived to have a different trajectory due to a difference in the time it takes for light to reach each eye.
Rubin's Vase (Figure-Ground)
An optical illusion that demonstrates the brain's ability to perceive either the vase (figure) or the faces (ground) in an image, but not both at the same time.
The tendency to remember the last items in a list or sequence more easily than those in the middle.
Serial Position Effect
The combination of the Primacy and Recency Effects: the tendency of a person to recall the first and last items in a series best, and the middle items worst. When asked to recall a list of items in any order (free recall), people tend to begin recall with the end of the list, recalling those items best.
Shepard Tone Illusion
A continuously ascending or descending audio pitch that seems to never reach a limit. Has been used in torture, and to create extreme discomfort in filmmaking.
Simultaneous Contrast Illusion
A color or shade is influenced by its surrounding colors or shades. For example, spots of paint on a white background appear almost black and conversely paler than their true colour on black.
The tendency for persuasive messages from non-credible sources to become more persuasive over time. First noticed among US Army soldiers exposed to army propaganda, who forgot that the message was propaganda.
The tendency for people to overestimate the extent to which others notice their actions and appearance; people tend to believe they are being noticed more than they really are.
A delay in reaction time between congruent and incongruent stimuli. The brain experiences interference in processing the color of a word when the word's meaning is a different color. For example, when the word "red" is written in blue ink, it takes longer for people to identify the ink color.
Sunk Cost Fallacy
The tendency to continue investing in a decision based on the amount already invested, rather than the current and future value. In for a penny, in for a pound; good money after bad; so much money in, the rest is justified; already invested too much to leave.
The brain fails to recognize facial distortions when a face is upside down. When the face is rotated 180 degrees, the distortions become apparent and unsettling.
The reduction of pressure when a fluid flows through a constricted section of a pipe can create illusions of suction or levitation. In inviscid fluid dynamics, an incompressible fluid's velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy (Bernoulli's principle). Thus, any gain in kinetic energy a fluid may attain by its increased velocity through a constriction is balanced by a drop in pressure.
Wagon-Wheel (Stroboscopic) Effect
A spoked wheel appears to rotate too slowly, or in the wrong direction, due to the sampling rate of the visual system or a camera.
Waterfall Illusion (Motion Aftereffect)
Stationary objects appear to be moving in the opposite direction after viewing a moving stimulus for an extended period. If one looks at a waterfall for about a minute and then looks at the stationary rocks at the side of the waterfall, these rocks appear to be moving upwards slightly.
The tendency to remember unfinished tasks or interrupted activities more easily than completed ones. Waiters have better recollections of still unpaid orders, but can't remember them after they've been paid.
Parallel lines that appear to be slanted due to the presence of short intersecting diagonal lines.