Rainbow
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A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the Sun. Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.
A colorful rainbow and ring-billed gull
Double rainbow and supernumerary rainbows on the inside of the primary arc. The shadow of the photographer's head at the bottom of the photograph marks the centre of the rainbow circle (the antisolar point).
Mathematical derivation
Double rainbow with Alexander's band visible between the primary and secondary bows. Also note the pronounced supernumerary bows inside the primary bow.
Dispersion (optics)
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In optics and in wave propagation in general, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to optics in particular.
A medium having this common property may be termed a dispersive medium.
A compact fluorescent lamp seen through an Amici prism