Questions: Wave-Particle Duality

Question 1

In a double-slit experiment, electrons are fired one at a time. After thousands of electrons have been detected, the pattern on the screen...

• A. Shows two bright bands directly behind each slit, confirming particles traveled one path
• B. Shows an interference pattern with multiple bright and dark fringes
• C. Shows a single central bright band regardless of slit spacing
• D. Shows no pattern — each electron lands at a completely random location

Answer: (B)

Even though each individual electron arrives as a localized click (particle behavior), the accumulated distribution over many electrons forms an interference pattern (wave behavior). This is only possible if each electron's wavefunction passes through both slits and interferes with itself. The pattern disappears if which-path detectors are added at the slits.

Question 2

In the double-slit experiment, each electron literally splits into two halves — one half going through each slit — and then recombines at the detector.

• T. True
• F. False

Answer: False

Electrons do not split. Each electron is detected as a single, whole, localized particle. What spreads through both slits is the electron's wavefunction — a probability amplitude, not a physical division of the electron's substance. The interference pattern reflects the wavefunction's behavior, but each detection event is one undivided electron.

Question 3

If you place detectors at the slits to determine which slit each electron passes through, what happens to the interference pattern?

Model answer: The interference pattern disappears, replaced by two plain bands corresponding to the two slits.

Measuring the electron's path collapses its wavefunction to a definite trajectory. Once the path is determined, the electron behaves like a classical particle passing through one slit, and there is nothing to interfere. This is Bohr's complementarity principle: wave behavior (interference) and particle behavior (definite path) are mutually exclusive — the experimental arrangement determines which aspect you observe.