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Diffraction through a gap, and the double-slit pattern

A distinct wave phenomenon from refraction or reflection: waves SPREAD OUT after passing through a gap or around an edge — the basis of the double-slit interference pattern.

What diffraction is, and when it is most noticeable

• DIFFRACTION: waves spread out after passing through a gap (or around an obstacle) — the narrower the gap RELATIVE to the wavelength, the MORE the waves spread.
• A gap much WIDER than the wavelength barely diffracts the wave — it mostly continues straight through, with only slight spreading at the edges.
• A gap comparable in size to (or narrower than) the wavelength diffracts strongly, spreading the wave out almost like a new point source at the gap.
• DOUBLE-SLIT PATTERN: light passing through TWO narrow, closely-spaced slits diffracts at EACH slit, and the two spreading waves then INTERFERE with each other, producing alternating bright (constructive) and dark (destructive) bands on a screen.

Worked example Explaining how a double-slit light pattern forms Purple light passes through a double slit and forms alternating bright and dark bands on a screen. Name the phenomenon and explain the conditions required for the pattern to form. NAME THE PHENOMENON: diffraction (at each slit) combined with interference (between the two resulting waves) — sometimes referred to together as double-slit interference. CONDITION 1 — DIFFRACTION AT EACH SLIT: each slit must be narrow enough (comparable to the light's wavelength) to diffract the light into a spreading wave, rather than a narrow beam. CONDITION 2 — COHERENT SOURCES: the two slits act as two wave sources that are IN PHASE with each other (since they come from the same original light source), which is required for a stable interference pattern to form. THE PATTERN: where the path difference from the two slits to a point on the screen is a WHOLE number of wavelengths, the waves arrive in phase (constructive, bright); where it is a half-integer number of wavelengths, they arrive out of phase (destructive, dark). ✓ Name BOTH phenomena involved (diffraction creating the spreading waves, then interference combining them) — naming only "interference" without explaining how each slit produces a spreading wave in the first place is an incomplete answer. ✕

Describing diffraction as identical to refraction

Diffraction (spreading through a gap, no change in medium) is a DIFFERENT phenomenon from refraction (bending due to a change in speed when entering a new medium). A gap in a barrier does not change the wave's speed or medium — it simply allows the wave to spread out geometrically.

Gap width vs wavelength	Diffraction effect
Gap much wider than λ	Minimal spreading — wave mostly continues straight
Gap comparable to or narrower than λ	Strong spreading — nearly a new point source

A fast check on a diffraction/double-slit answer

• Have I named BOTH diffraction (spreading at each slit) AND interference (the two waves combining), not just one?
• Have I linked the gap width to the wavelength explicitly, rather than saying "narrow" or "wide" alone?

Practice for this standard covers the whole topic — flashcards and quiz.