Drop two stones into a pond and watch the ripples meet. Where crests meet crests, the water rises higher. Where crests meet troughs, the water flattens. That is interference, the combination of waves that can reinforce or cancel each other. Constructive interference happens when waves are in phase, meaning their peaks align. The result is a larger amplitude. Destructive interference happens when waves are out of phase, meaning peaks align with troughs. The result is a smaller amplitude or no wave at all.

Interference happens with all waves, including light, sound, and water. It is the basis of many technologies. Noise-canceling headphones produce sound waves that destructively interfere with ambient noise. Antireflective coatings on lenses use interference to reduce reflections. Interferometers use interference to measure tiny distances, detect gravitational waves, and test optical surfaces. The famous double-slit experiment demonstrates that light and matter can interfere with themselves, which is one of the strangest and most important results in quantum mechanics. Interference is not a flaw. It is a fundamental property of waves.

Interference facts

• Definition. Combination of waves.
• Constructive. Waves in phase, amplitude increases.
• Destructive. Waves out of phase, amplitude decreases.
• Applications. Noise-canceling headphones, antireflective coatings, interferometers.
• Double-slit. Demonstrates wave-particle duality.

Interference is not the same as diffraction. Diffraction is the spreading of waves. Interference is the combination of waves.