It is formed due to scattering of light through moisture present in air. When white light is passed through water droplets present in air, it gets dispersed into seven colours i.e, violet,indigo,blue,green,yellow,orange and red.Actually rainbow is formed around the earth that means it is round in shape. But due to our restricted eye focal length we can see it  in the shape of downward parabola.
1 5 1
Rainbow is formed when sunlight spread out intothe spectrum of colors ie VIBGYOR  and diverted to the eye of the observerby water droplets .its a group of circular arcs of colors,all having common is the outermost color and violet is the innermost.All other colors are inbetween its arranged as such due to the difference in their wavelength. Higher the wavelengthlower the deviation
1 5 1