A level Physics · Edexcel 9PH0 · Topic 5D

Diffraction and interference

Key points

Diffraction is the spreading of a wave as it passes through a gap or around an obstacle; it is greatest when the gap is about the same size as the wavelength.
Coherent sources have the same frequency and a constant phase difference, needed for a stable interference pattern.
In a two-source pattern, bright fringes (maxima) occur where the path difference is a whole number of wavelengths.
A diffraction grating has many closely spaced slits and produces sharp, bright maxima at angles given by $n\lambda = d\sin\theta$ , where $d$ is the slit spacing.
To find $d$ from lines per metre $N$ : $d = \frac{1}{N}$ . The highest order seen is the largest whole number $n$ with $\frac{d}{\lambda} \geq n$ .
Electron diffraction through a thin layer of graphite gives rings, evidence of the wave nature of electrons.
Core practical: determine the wavelength of laser light using a diffraction grating, measuring the angle to each order.

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Waves spread out after passing through gaps, and overlapping waves from coherent sources produce stable patterns. These notes cover diffraction, path difference, the diffraction grating, electron diffraction and core practical 8.

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