Interference in science most commonly refers to what happens when two or more waves meet at the same point in space. The waves combine, and depending on how they align, they can amplify each other, cancel each other out, or produce something in between. This principle applies to light, sound, water waves, and even subatomic particles. The term also shows up in psychology and biology with related but distinct meanings.
The Core Idea: Waves Combining
When two waves arrive at the same location, their disturbances add together. This is called superposition. Each wave produces a force, and those forces stack on top of one another. If both waves push in the same direction, you get a bigger wave. If they push in opposite directions, they reduce or eliminate each other. That addition (or subtraction) of wave energy is interference.
There are two pure forms. Constructive interference happens when two waves arrive perfectly in phase, meaning their peaks line up with peaks and their troughs line up with troughs. The result is a combined wave with twice the amplitude of either individual wave. Destructive interference is the opposite: one wave’s peak arrives exactly at the other’s trough. The two cancel out completely, producing zero amplitude.
Most real-world interference falls somewhere between these extremes, with waves partially reinforcing or partially canceling depending on how closely their phases align.
Young’s Double Slit Experiment
The most famous demonstration of interference came in 1801, when English physicist Thomas Young shone light through two narrow slits and observed the pattern it created on a screen behind them. Without interference, the light would have simply formed two bright lines. Instead, it produced a series of alternating bright and dark bands called interference fringes.
The bright bands appeared where light waves from the two slits arrived in phase, reinforcing each other through constructive interference. The dark bands appeared where the waves arrived out of phase, canceling through destructive interference. Specifically, constructive interference occurs wherever the difference in distance traveled by the two waves equals a whole number of wavelengths. Destructive interference occurs where that difference equals a half wavelength, one and a half wavelengths, and so on.
This experiment was the definitive proof that light behaves as a wave. It remains one of the most important experiments in physics, and versions of it continue to reveal surprising things about quantum mechanics.
Quantum Interference: Particles Acting Like Waves
The double slit experiment becomes genuinely strange when you run it with individual particles instead of a beam of light. Fire single electrons or single photons at a double slit one at a time, and each one lands at a specific point on the detector, like a particle would. But as thousands of individual hits accumulate, the familiar interference pattern of bright and dark bands gradually emerges.
Each particle somehow “interferes with itself,” as physicist P.A.M. Dirac put it. It behaves as though it passed through both slits simultaneously as a wave, then arrived at the detector as a particle. This self-interference is a direct consequence of quantum superposition.
Here’s the truly puzzling part: if you set up a detector to determine which slit each particle actually passes through, the interference pattern disappears. The act of measuring the path information destroys the wave behavior. This is the heart of wave-particle duality, and it remains one of the deepest puzzles in physics.
Interference You Can See and Hear
Interference isn’t just a laboratory concept. The rainbow colors swirling across a soap bubble are a direct result of thin-film interference. Light reflects off both the outer and inner surfaces of the soap film, and those two reflected waves interfere with each other. Because the film’s thickness varies across the bubble, different wavelengths (colors) get constructively reinforced at different spots, producing the shifting rainbow pattern. The same physics explains the iridescent colors on oil slicks and the coatings on camera lenses.
Noise-canceling headphones are a practical application of destructive interference. A small microphone picks up ambient sound, and the headphone’s speaker emits the same sound wave but with its phase inverted, so peaks align with troughs. When the original noise and the inverted copy combine in your ear canal, they cancel each other out, reducing the volume of background noise. Adaptive algorithms continuously analyze incoming sound and adjust the cancellation signal in real time.
Electromagnetic Interference
In electronics, interference takes on a more practical and often unwanted meaning. Electromagnetic interference (EMI), sometimes called radio-frequency interference, is any disturbance from an external source that disrupts an electrical circuit. The disruption can travel through electromagnetic radiation, electrostatic coupling, or direct conduction along wires. Effects range from a slight increase in data errors to a complete loss of signal.
Sources are everywhere, both natural and human-made. Lightning, solar flares, and auroras all generate electromagnetic disturbances. On the human side, power lines, mobile phone networks, Wi-Fi routers, switched-mode power supplies, arc welders, and even microcontrollers all emit energy that can bleed into nearby circuits. AM radio is particularly susceptible, but FM radio, television, and sensitive scientific instruments are also affected. In one notable incident, a solar flare disrupted and distorted a recording of a U.S. House of Representatives debate in 2002.
Interference in Memory and Psychology
Psychology uses “interference” to describe how memories can disrupt one another. There are two types, and they work in opposite directions.
Proactive interference happens when something you already know makes it harder to remember new information. If you memorized your old phone number years ago and keep accidentally recalling it instead of your new one, that’s proactive interference. The older memory is “protecting” itself at the expense of the newer one.
Retroactive interference is the reverse: new learning disrupts your ability to recall something older. Learning a new language, for example, can temporarily make it harder to retrieve vocabulary from a language you studied years ago. The new information overwrites or competes with the original memory during the consolidation process. Research on memory consolidation shows that timing matters: whether a new memory disrupts an old one (or vice versa) depends on when during the consolidation window the second learning event occurs.
RNA Interference in Biology
In molecular biology, RNA interference (RNAi) is a natural process cells use to silence specific genes. It works by destroying the messenger molecules that carry a gene’s instructions to the protein-building machinery of the cell.
The process has two main steps. First, a long strand of double-stranded RNA gets chopped into small fragments roughly 21 to 25 units long, called small interfering RNAs. Second, these fragments join a protein complex that hunts for matching messenger RNA in the cell. When it finds a match, the complex cuts the messenger RNA roughly in the middle, effectively preventing that gene’s instructions from being carried out. The chopped-up messenger RNA is then broken down further by the cell’s cleanup enzymes.
RNAi is both a natural defense mechanism (cells use it to fight off viral RNA, for instance) and a powerful research tool. Scientists can introduce custom-designed RNA fragments into cells to selectively shut down specific genes and observe what happens, making it one of the most important techniques in modern genetics.