Waves do not only travel in straight lines: passing through a gap or round an obstacle they spread out. How much they spread depends on the size of the gap compared with the wavelength.
Diffraction is the spreading of a wave as it passes through a gap or around the edge of an obstacle. The spreading is greatest when the gap width is comparable to the wavelength. A wide gap (much larger than the wavelength) gives little spreading; a gap close to one wavelength gives strong spreading. Diffraction is a property of all waves.
Change the gap width relative to the wavelength and watch the diffracted wavefronts spread. The spreading becomes dramatic as the gap shrinks toward one wavelength.
The ratio of gap width to wavelength controls the spreading.
| Gap width | Diffraction |
|---|---|
| gap much wider than λ | very little spreading |
| gap a few wavelengths wide | moderate spreading |
| gap about one wavelength | strong spreading |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
Diffraction is the:
Diffraction at a gap is most pronounced when the gap width is:
When a wave passes through a gap much wider than its wavelength, it:
Diffraction is a property of:
Everyday diffraction comes from matching wavelengths to gap sizes.
Diffraction does not change the wavelength, frequency or speed of the wave; it only changes its direction of spread. The spreading is greatest when the gap is comparable to the wavelength, not when it is far smaller or far larger. And diffraction happens for all waves, which is why both sound and light diffract, just by different amounts.
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Water waves of wavelength 2 cm pass through a gap. The greatest spreading occurs when the gap width is about:
Sound diffracts around a doorway far more obviously than light does because:
As a gap is made narrower toward one wavelength, the diffracted waves spread:
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