Rainbows are an optical phenomenon formed by the refraction, internal reflection, and dispersion of light in water droplets, which consistently emerge at a characteristic angle of approximately 42 degrees relative to the anti-solar point for the primary bow. This is a matter of basic physical optics and everyday observation requiring no formal citation.
The claim states a fundamental, well-understood law of geometrical optics regarding atmospheric rainbows (the primary rainbow's ~42-degree angle). By the instructions, statements that are trivially true by definition or direct everyday observation are categorized as COMMON_KNOWLEDGE, with no citations or paper indices attached.