Various atmospheric optical phenomena are produced by different combinations of light-matter interaction processes: rainbows result from refraction, internal reflection, and wavelength-dependent dispersion of sunlight within spherical water droplets; mirages result purely from refraction of light through air layers of differing density (temperature-dependent refractive index gradients); coronae (bright rings close around the sun or moon) result from diffraction of light around small water droplets or ice crystals; and haloes result from refraction, reflection, and dispersion of light within ice crystals suspended in the atmosphere (analogous to the rainbow process but occurring in ice rather than water).