<Dl> <Dd> sin (2β − φ) = n sin β, </Dd> </Dl> <Dd> sin (2β − φ) = n sin β, </Dd> <P> where n = 1.333 is the refractive index of water . Solving for φ, we get </P> <Dl> <Dd> φ = 2β − arcsin (n sin β). </Dd> </Dl>

Which is the first colour in the rainbow