diffraction
/ dif-RAK-shun /
Look at a streetlight at night through the weave of a thin curtain and you see it smear into a cross or a pattern of spots, not a single point. Waves do not simply pass through small openings or past edges — they bend and fan out and overlap. That bending and the patterned interference it creates is diffraction.
More precisely, diffraction is the spreading and interference of waves when they encounter an obstacle or opening comparable in size to their wavelength. As the wavefronts bend around edges and recombine, they reinforce in some directions and cancel in others, producing a characteristic pattern of bright and dark regions. It happens for light, sound, water waves, and the wave-like behaviour of electrons.
Why it matters: diffraction is the physical heart of how we probe structure at the atomic scale. Because the pattern depends precisely on the spacing of whatever the waves bounced off, measuring it lets us read out spacings far too small to see. A caveat: clear, useful diffraction patterns appear only when the wavelength is comparable to the spacing — which is why X-rays and electrons, not visible light, are used to see atoms.
Shine a laser pointer through a fine wire mesh and the dot on the wall blossoms into a grid of dots. The spacing of those dots depends on the spacing of the mesh — squeeze the mesh finer and the dots spread wider. The same physics, with X-rays and a crystal, lets chemists measure the spacing between layers of atoms.
The geometry of a diffraction pattern encodes the spacing of whatever scattered the waves.
Diffraction also sets a hard limit on ordinary microscopes: you cannot clearly see features much smaller than the wavelength of the light you use. This is precisely why electron microscopes, which use electrons of far shorter wavelength, can resolve detail that light microscopes never can.