The experiment that revealed the wave-particle duality at the heart of quantum mechanics.
When waves pass through two slits, they create an interference pattern. Regions where crests meet crests produce bright fringes (constructive interference), while crests meeting troughs create dark fringes (destructive interference).
Here, $d$ is the slit separation, $\theta$ is the angle to the fringe, $\lambda$ is the wavelength, and $m$ is the order number.
Drop two pebbles in a calm pond. The circular ripples overlap, creating regions where the water is calm (destructive) and regions where waves are amplified (constructive).
When we send individual photons or electrons through the double slit one at a time, each particle creates a single dot on the detector. But over time, these dots accumulate into the same interference pattern—as if each particle "interferes with itself."
This challenges our classical intuition: How can a single particle go through both slits simultaneously?
Until measured, the particle exists in a superposition—simultaneously taking both paths. The act of measurement collapses this superposition to a definite state.
If we try to determine which slit the particle goes through by placing a detector, the interference pattern disappears! The particle suddenly behaves like a classical particle, creating two separate bands instead of an interference pattern.
This demonstrates that observation fundamentally alters the quantum system. Measurement collapses the wave function, forcing the particle to "choose" a definite path.
The experiment suggests that at the quantum level, reality is not independent of observation. The act of measurement plays an active role in determining the outcome.