Diffra

Meet Diffra, light passing through two slits that paints stripes on the far wall.

Thomas Young showed around 1801 that light passing through two narrow slits makes a pattern of bright and dark bands on a screen. Waves from the two slits overlap. Where the extra distance travelled by one, d sin θ, is a whole number m of wavelengths, crests meet crests and the light is bright. Halfway in between, crests meet troughs and cancel to dark. This was powerful evidence that light is a wave.

Example: red laser light of 650 nanometres through slits 0.25 mm apart makes the first bright fringe at sin θ ≈ 0.0026. On a screen 2 m away, the fringes are spaced about 5 millimetres apart.

The same idea underlies diffraction gratings in spectrometers and interferometers, some of the most precise measuring tools in physics.

Done one particle at a time with electrons or single photons, the pattern still builds up, a central puzzle of quantum mechanics. Real slits have width, so the fringes also fade under a broader single-slit envelope.