What did Young's double-slit experiment originally demonstrate about light?

That light is a particle — each slit produces a separate bright band That light is a wave — it produces an interference pattern of alternating bright and dark bands that only wave superposition can explain That light travels in straight lines and cannot bend around obstacles That light has no wavelength — it behaves identically to sound

What happens when a single photon is fired at the double-slit apparatus?

It goes through one specific slit and lands in a predictable spot Nothing — single photons are too small to interact with the slits It produces a single dot on the detector — but after thousands of single-photon shots, the accumulated dots form an interference pattern It goes through both slits simultaneously and creates two dots on the detector

What happens to the interference pattern if you place a detector at the slits to determine which slit the photon went through?

The interference pattern becomes stronger — knowing the slit's position reinforces the waves The interference pattern vanishes — you get two simple bright bands as if the photons are classical particles The pattern stays the same — the detector doesn't affect the photon The photon refuses to go through either slit — it bounces back at the detector

Does wave-particle duality only apply to photons?

Yes — only massless particles like photons can interfere with themselves No — electrons, neutrons, atoms, and even large molecules show the same interference behaviour in double-slit experiments No — only waves in water show true interference; light just mimics this behaviour Yes — massive particles always behave as classical particles in double-slit experiments

Which condition gives a bright fringe in double-slit interference?

Path difference = (n + ½)λ — half-wavelength offset causes maximum constructive addition Constructive interference: path difference from the two slits = nλ (whole number of wavelengths) — crests arrive in phase Both slits must be blocked — bright fringes appear when no light passes through The path difference must be zero — only the central bright fringe exists

A double-slit experiment is done with light of wavelength 500 nm. If the slit separation is halved, what happens to the fringe spacing?

Fringe spacing halves — closer slits produce denser fringes Fringe spacing is unchanged — it depends only on wavelength and screen distance Fringe spacing doubles — y = λL/d, so halving d doubles y Fringe spacing quadruples — d appears squared in the formula

What does the wave function (Ψ) describe in the context of the double-slit experiment?

The physical wave that carries the particle through space, like a water wave carries a boat The quantum amplitude — a complex-valued field that encodes the probability of finding the particle at each location via |Ψ|² The electric field of the photon The trajectory of the particle through the slits

Why can't we explain the double-slit results by saying 'the photon goes through one slit but we just don't know which one'?

We can — the photon does go through one slit; the interference is caused by the edges of the slits Because experiments where we DO measure which slit destroy the interference — if the photon truly went through one slit, interference could not occur even when we don't measure We can't explain it at all — double-slit interference is still a complete mystery with no theory Because photons go through both slits at different times and then combine on the screen

Richard Feynman said the double-slit experiment contains 'the only mystery' of quantum mechanics. What did he mean?

Feynman meant that the slits are very difficult to manufacture precisely — the mystery is engineering, not physics All of the puzzling features of quantum mechanics — superposition, measurement, wave-particle duality — are contained in this single experiment; understand it deeply and you understand QM Feynman meant that nobody has ever performed the single-photon version — it remains theoretical The mystery is why light chooses red and violet bands — the colour selection has no explanation

An electron travelling at 2 × 10⁶ m/s has a de Broglie wavelength λ = h/mv. Given m = 9.1 × 10⁻³¹ kg and h = 6.6 × 10⁻³⁴ J·s, which range does λ fall in?

About 0.36 nm — in the X-ray range, comparable to atomic spacings in crystals About 500 nm — the same as visible light About 1 km — too large to show interference with any physical slit About 1 fm (10⁻¹⁵ m) — nuclear scale