How Does Single-Slit Diffraction Work and What Does Its Intensity Pattern Look Like? (HL)

Single-slit diffraction occurs when a wave passes through a narrow slit and spreads out because wavelets from different parts of the slit interfere. The resulting intensity pattern has a wide, bright central maximum surrounded by narrower, progressively weaker secondary maxima.

The mechanism

According to the principle of superposition, every point across a slit of width $b$ acts as a source of coherent wavelets. At the centre of the screen, all wavelets travel equal distances and arrive in phase, producing maximum constructive interference.

Away from the centre, wavelets from different parts of the slit travel different distances. At particular angles, they cancel completely, producing intensity minima. The minima satisfy

b\sin\theta=n\lambda, \qquad n=1,2,3,\ldots

How Does Single-Slit Diffraction Work and What Does Its Intensity Pattern Look Like? (HL)

The mechanism

Away from the centre, wavelets from different parts of the slit travel different distances. At particular angles, they cancel completely, producing intensity minima. The minima satisfy

b\sin\theta=n\lambda, \qquad n=1,2,3,\ldots

Pattern feature	Explanation
Central maximum	Brightest region, extending between the first minima on either side
Central width	Approximately twice the width of each secondary maximum
Secondary maxima	Much less intense and become progressively weaker away from the centre
Minima	Zero intensity ideally, occurring when $b\sin\theta=n\lambda$
Symmetry	The pattern is symmetric about the central axis

How Does Single-Slit Diffraction Work and What Does Its Intensity Pattern Look Like? (HL)

The mechanism

How Does Single-Slit Diffraction Work and What Does Its Intensity Pattern Look Like? (HL)

The mechanism

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Exam technique