diffraction
/ dih-FRAK-shun /
Drop a stone in a pond and watch the ripples spread out in smooth rings. Now imagine the ripples meeting a row of evenly spaced posts: each post sends out its own little circle of waves, and these overlap. In some directions the crests line up and the wave grows tall; in others a crest meets a trough and they flatten out. That bending-and-overlapping of waves around a regular array is diffraction.
More precisely, diffraction happens whenever a wave passes through or scatters off objects spaced about as far apart as the wave is long. Each object becomes a tiny new source of waves, and the many wavelets add together — a process called interference. The result is not a uniform glow but a sharp pattern of bright and dark, fixed entirely by the wavelength and the spacing of the objects. Make the spacing regular, like atoms in a crystal, and the bright regions sharpen into crisp spots.
This matters because diffraction is how we 'see' things far too small for any lens: the pattern encodes the hidden spacings. It is also why a lens can never resolve detail finer than the wavelength of the light it uses. A common confusion: diffraction is not the wave being reflected or absorbed — it is many scattered wavelets reinforcing in some directions and cancelling in others. No single wave 'knows' the pattern; the pattern emerges only from the whole collection adding up.
Hold a CD up to a lamp and tilt it: rainbow stripes fan out across its surface. The disc's microscopic tracks are spaced about as far apart as visible light is long, so the reflected light diffracts, with each color reinforced at its own angle — the same physics that reveals atoms, just at a scale your eye can catch.
A CD's evenly spaced tracks diffract white light into rainbows.
To get sharp spots you need a wavelength comparable to the spacing. Visible light, far longer than atomic spacings, only diffracts off things like CDs or feathers; to probe atoms you need much shorter waves such as X-rays.