Diffraction

When a wave passes through a gap or around an obstacle, it spreads out. This spreading is called diffraction, and it is one of the most compelling demonstrations that light behaves as a wave. While single-slit diffraction produces a broad central maximum flanked by weaker subsidiary maxima, diffraction gratings produce sharp, well-separated maxima that are immensely useful in spectroscopy.

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What Is Diffraction?

Diffraction is the spreading of a wave as it passes through a gap (aperture) or around an obstacle. All waves diffract, but the effect is most noticeable when the size of the gap or obstacle is comparable to the wavelength of the wave.

• If the gap is much larger than the wavelength, the wave passes through with very little spreading — diffraction is negligible.
• If the gap is comparable to the wavelength, the wave spreads significantly after passing through.
• If the gap is much smaller than the wavelength, the wave spreads out almost uniformly in all directions, like a circular wavefront emerging from a point source.