Interference occurs whenever two or more waves, signals, or processes overlap and either strengthen or weaken each other. In physics, the classic answer is that two waves from coherent sources traveling different path lengths will interfere when they meet. But interference shows up across many fields, from the colors on a soap bubble to the way old memories block new ones. Here’s a breakdown of the specific situations that produce interference and what determines whether the result is stronger or weaker.
Wave Interference in Physics
The most common context for this question is a physics class, where interference refers to two waves occupying the same space at the same time. For interference to happen, you need two key conditions: the waves must come from sources with the same frequency, and their overlapping paths must create a consistent phase relationship. This is called coherence. Two random, unrelated sources won’t produce a stable interference pattern.
Once those conditions are met, the type of interference depends on how the path lengths compare. Constructive interference happens when the difference in distance traveled by the two waves equals a whole number of wavelengths (0, 1, 2, 3, and so on). In this case, the peaks of both waves line up, and the combined wave is stronger. Destructive interference happens when that path difference equals a half wavelength plus any whole number of wavelengths. Now the peak of one wave lines up with the valley of the other, and they cancel out partially or completely.
So the direct answer: any situation where two coherent waves meet after traveling different distances will result in interference. The specific outcome (louder or quieter sound, brighter or darker light) depends on whether that distance difference is a whole number of wavelengths or offset by half a wavelength.
Thin Film Interference: Soap Bubbles and Oil Slicks
One of the most visible real-world examples is thin film interference, which creates the rainbow colors on soap bubbles and oil puddles. Light hits the front surface of the film and some reflects back. The rest passes through and reflects off the back surface. These two reflected beams then overlap and interfere.
The critical factor is the thickness of the film relative to the wavelength of light. When the film is one-quarter of a wavelength thick, the light traveling through the film covers an extra half wavelength compared to the front-surface reflection. This puts the two reflections in phase, producing a bright reflection for that color. At the very top of a soap bubble, where the film is extremely thin compared to any wavelength, the two reflections cancel out and the film looks dark. As thickness increases, the pattern alternates between bright and dark every quarter wavelength. Because different colors have different wavelengths, a film of varying thickness reflects different colors at different spots, creating the familiar swirling rainbow pattern.
Signal Interference in Electronics
Electromagnetic interference happens when radio waves, electrical signals, or other electromagnetic energy disrupts a device’s normal operation. Wi-Fi routers, Bluetooth devices, cellular networks, and satellites all emit radio frequency energy. When too many of these signals overlap in the same frequency range, the result is dropped calls, slower data speeds, or poor GPS accuracy.
Urban environments are particularly prone to this. Signals reflect, refract, and diffract off buildings and surfaces, creating localized hotspots of electromagnetic energy. The explosion of internet-connected devices has made this worse, with billions of low-power transmitters collectively raising the electromagnetic density in homes and offices. Even a microwave oven can interfere with a Wi-Fi signal because both operate near the 2.4 GHz frequency band.
5G and Aircraft Altimeters
A high-profile example involved 5G cellular signals interfering with aircraft radar altimeters, which measure altitude during landing. The 5G C-band frequencies sit close to the frequencies altimeters use, and there was concern that powerful 5G towers near airports could degrade altimeter readings. The FAA worked with wireless carriers like Verizon and AT&T to delay some C-band deployment until airlines could retrofit their equipment. By the end of September 2023, the entire U.S. airline fleet had upgraded, and the interference risk was considered mitigated. The FAA now requires transport aircraft in the U.S. to carry 5G-tolerant altimeters or radio frequency filters.
Memory Interference in Psychology
Interference isn’t limited to physics. In psychology, it describes what happens when memories compete with each other and make recall harder. There are two directions this can go.
Proactive interference occurs when older memories block the learning or recall of newer information. If you memorized a phone number years ago and then switched carriers, the old number might keep popping into your head when you try to remember the new one. Retroactive interference works the other way: newly learned information disrupts your ability to recall something older. After memorizing your new phone number, you might struggle to remember the old one.
Both types have been studied extensively. Older adults tend to be more susceptible to proactive interference, with research showing they commit roughly twice as many “previous list” memory errors as younger adults in recall experiments. Younger adults, interestingly, show a different vulnerability. In one study, when asked to recall items from the most recent list, younger adults actually recalled more words from earlier lists than from the current one, a clear sign of retroactive interference. The practical takeaway: studying similar material back to back (two foreign languages, two similar textbook chapters) increases the chance that one set of information will interfere with the other.
Cognitive Interference: The Stroop Effect
Your brain also experiences interference when two competing pieces of information demand a response at the same time. The most famous demonstration is the Stroop effect. If you see the word “RED” printed in blue ink and are asked to name the ink color, your response will be slower and more error-prone than if the word and color matched. Your brain’s reading pathway is so well-practiced that it fires automatically, competing with the weaker color-naming pathway.
This happens because word reading has stronger neural connections due to years of habitual practice, while color naming requires more deliberate effort. The conflict plays out in brain regions responsible for selective attention and response selection. Interference occurs both at the perceptual stage, where your brain processes the color, and at the response stage, where it selects which answer to give. The Stroop task is widely used in clinical settings to assess cognitive flexibility and executive function.
Multitasking creates a similar form of cognitive interference. When people switch between two tasks that require the same mental resources, response times slow measurably. In one study, participants performing a continuous dual task were significantly slower (about 530 milliseconds per response) compared to single-task performance (about 514 milliseconds). That gap may sound small, but it compounds across hundreds of decisions per hour, which is why texting while doing anything else feels so disruptive.
Genetic Crossover Interference
In genetics, interference describes something that happens during cell division when chromosomes exchange segments of DNA. Normally, these “crossover” events could occur at random positions along a chromosome. But in most species, when one crossover happens, it suppresses the likelihood of another crossover forming nearby. This is crossover interference, and it means crossovers end up more evenly spaced than random chance would predict.
One leading explanation is competitive sequestration: the molecular machinery needed to complete a crossover gets locally depleted after one event, starving nearby sites of the proteins they need. Another model compares it to a brittle film on a heated metal beam. Mechanical stress builds along the chromosome, and a crossover relieves that stress locally, preventing additional “cracks” from forming close by. Both models explain the same observation: one crossover inhibits others, with the effect fading over distance.
RNA Interference in Biology
At the molecular level, RNA interference is a natural defense mechanism that cells use to silence specific genes. It works in two steps. First, an enzyme called Dicer chops double-stranded RNA into small fragments roughly 21 to 25 units long. These small fragments then join a protein complex called RISC, which uses the fragment as a guide to find and destroy matching messenger RNA in the cell. Without that messenger RNA, the gene’s instructions never get carried out, effectively silencing it. Cells use this process to defend against viruses and regulate their own gene activity, and researchers have harnessed it as a powerful tool for studying gene function.