6.12 Single-Slit Diffraction

Diffraction: bending of waves

When light passes through a single slit comparable in size to its wavelength, it spreads out and creates a diffraction pattern on a screen — a bright central maximum flanked by dimmer fringes, unlike the equally bright fringes of double-slit interference. This happens because every point across the slit acts as a source of secondary wavelets that interfere with each other.

Locating dark fringes

For single-slit diffraction, dark fringes (minima) occur where a sin θ = mλ, with a = slit width, θ = angle from center, λ = wavelength, and m = ±1, ±2, ±3... Note m ≠ 0 gives no minimum at center — the center is always the brightest point, called the central maximum.

Slit width and pattern width

A narrower slit produces a wider diffraction pattern (fringes spread out more), while a wider slit produces a narrower pattern that approaches a simple bright spot. This inverse relationship comes directly from a sin θ = mλ: smaller a requires larger θ for the same m.

Key terms

Single-slit diffraction

The spreading of light passing through a narrow slit, producing a pattern with a bright central maximum and dimmer side fringes.

Condition for dark fringes (single slit)

a sin θ = mλ, where a = slit width, m = ±1, ±2, ±3...

Central maximum

The brightest and widest fringe in a diffraction pattern, located directly opposite the slit at θ = 0.

Effect of slit width on pattern

Narrower slit → wider diffraction pattern; wider slit → narrower diffraction pattern.

Why diffraction occurs (Huygens' principle)

Every point in the slit acts as a source of secondary wavelets; their interference creates the diffraction pattern.

Try an AP-style question

The width of a single slit is decreased. What happens to the central bright band?

1. It gets narrower
2. It stays the same
3. It disappears
4. It gets wider