The Central Mystery
If you fire electrons one by one at two slits, they hit the screen as individual "particles" (dots). But over time, the dots build up a Wave Interference Pattern. This is the Double Slit Experiment.
It means that each individual electron somehow passes through both slits at once and interferes with itself!
Worked Examples
Example 1: The Observed Electron
If you place a detector at the slits to "see" which one the electron goes through, the interference pattern disappears! The act of observation "collapses" the wave behavior, and the electron acts like a simple classical particle.
The Bridge to Quantum Mechanics
The Double Slit experiment is the reason we need the Wavefunction \(\psi\). We don't track the particle's path; we calculate the "Probability Amplitude" wave. The interference pattern is exactly the squared magnitude of the sum of the two paths: \(P = |\psi_{slit1} + \psi_{slit2}|^2\). This experiment proves that Quantum Mechanics is not just a theory of "small things," but a theory of how reality itself is constructed from waves of possibility.