Here A, B, C, represent three drops of rain in the primary (or inner) rainbow.
The least refracted line is red, and blue the most.
So the red (or least refracted rays) of all the drops in the position of A,—the yellow of those in the position of B,—and the blue (or the most refracted rays) of the lowest drops, all meet the eye D, and form a rainbow to the spectator.
The reason why the primary bow exhibits the stronger colours is this—because the colours are seen after one reflection and two refractions; but the colours of the secondary (or upper) rainbow undergo two reflections and three refractions.
Here also the least refracted ray is red, and the most refracted blue (as in the former case); but the position of each is reversed.
Q. Why does a soap bubble exhibit such variety of colours?
A. The changing colour of the bubble depends upon the changing thickness of the film through which the ray passes.
Q. How does the thickness of the film affect the colour of the soap bubble?
A. Because different degrees of thickness produce different angles of refraction, and, therefore, different colours reach the eye.
Q. Why is the soap bubble so constantly changing its thickness?