Diffraction

Diffraction — the spreading out of a wave as it passes through a gap or around an obstacle.

What changes and what does not — the wave spreads into the geometric shadow, but its wavelength, frequency and speed are all unchanged. Diffraction alters the shape of the wavefronts, not the wave itself.

The size of the gap matters — the amount of spreading depends on the ratio of the wavelength to the gap width.

Gap much larger than the wavelength: very little noticeable diffraction, and the wave continues almost as a beam.

Gap comparable to the wavelength: maximum spreading, with the wave fanning out in a semicircular pattern.

Example: this is why you can hear someone around a corner but not see them. Sound has a wavelength of around a metre, comparable to a doorway, so it diffracts strongly. Visible light has a wavelength of under a micrometre, so a doorway is over a million times wider than its wavelength and it barely spreads at all.

Single-slit diffraction pattern — monochromatic light through a single narrow slit gives a pattern of a wide, bright central maximum with dimmer maxima either side, separated by dark minima. The central maximum is about twice the width of the others and far brighter.

Changing the conditions

Narrower slit — the pattern spreads out, so the central maximum becomes wider.

Longer wavelength — also spreads the pattern, so red light gives wider fringes than blue.

White light — the central maximum is white, since all wavelengths overlap there, with coloured fringes either side because each wavelength spreads by a different amount.

Diffraction and evidence for wave behaviour — diffraction is a property only waves display, so observing it is evidence that whatever is being studied is behaving as a wave. Electrons diffracted by a crystal lattice show the same pattern, which is why electron diffraction is taken as evidence for the wave nature of particles.