Each of these everyday sights is produced by a different behaviour of light. Watch the four visualisations below — aurora, the blue polarised sky, the rainbow, and interference fringes — and read why each one is matched to its cause.
Charged particles from the Sun (the solar wind) are steered by Earth's magnetic field toward the poles. They crash into oxygen and nitrogen atoms high in the atmosphere and knock their electrons to higher energy levels. When those electrons fall back, the atoms emit light — green and red from oxygen, blue-violet from nitrogen. No reflection or bending of sunlight is involved; the atmosphere itself glows. → Emission of radiation (5).
Sunlight arrives unpolarised — its electric field wobbles in every direction. Air molecules absorb and re-radiate it (Rayleigh scattering). A molecule cannot radiate along its own oscillation axis, so light scattered at about 90° to the Sun comes out vibrating in essentially one plane. That is why the blue sky, viewed at a right angle to the Sun, is partially polarised. → Scattering by molecules (4).
A ray of sunlight enters a spherical raindrop and refracts. Because each colour bends by a slightly different amount, the white light spreads into a spectrum — this is dispersion. The separated colours strike the back of the drop and reflect (total internal reflection), then refract once more on the way out, leaving at about 42°. The drops across the sky send these colours to your eye as an arc. → Dispersion and reflection (1).
When light passes through a narrow slit or aperture it bends around the edges and spreads out — diffraction. The spread-out wavelets then overlap: where crest meets crest the light adds up to a bright fringe, where crest meets trough they cancel to a dark fringe. The regular pattern of dark and bright bands is the fingerprint of diffraction (and interference). → Diffraction (3).