SOME rainbows - like Monday mornings, rays of hope, and young men's prospects' when they go a-wooing - are brighter than others. Their appearance depends very much, on the size of the raindrops that reflect the sunlight. Very "small drops give a wide and" nearly colourless rainbow, while the large raindrops of a heavy shower produce a narrow, bright bow with sharp edges and colours well defined.
But look more closely at a rainbow and you will notice something else of interest: the sky beneath the bow is brighter than the sky outside. To understand why, we must recall again why we see a rainbow in the first place.
Millions of raindrops, acting like tiny mirrors, form a rainbow. Rather than being reflected on the surface, however, the light passes into each drop to be reflected from the back, the angle of reflection being, in general, 42 degrees. Moreover, the sunlight, as it passes from air to water and out again, is separated into its constituent colours: as the eye picks up different colours from different strategically placed drops, we see the familiar rainbow pattern with violet on the inside and red on the outer edge, forming a segment of a circle with an angular radius of 42 degrees.
The brightness of the sky inside the bow occurs because most but not all, of the light reflected inside a raindrop emerges at the critical angle of 42 degrees. A small proportion of the light rays which hit each drop are returned at somewhat smaller angles. Our eyes, therefore, can receive these "maverick" rays from any of the drops located on the violet side of the rainbow - or inside the bow - and this adds the general brightness of the sky in the vicinity.
Then again, sometimes when a ray of sunshine is reflected from the far side of a raindrop, not all the light succeeds in making its escape. Some of it is reflected a second time from the inner surface of the sphere of water, and only emerges from the raindrop at its next attempt. The geometry is such that this twice reflected ray of light finally emerges from the drop at an angle of 51 degrees to the original ray of sunlight.
The result is that we sometimes see a larger secondary rainbow, concentric with the main, resulting from raindrops strategically placed so that our eyes can catch the light emerging from them at an angle of 51 degrees to the original sunbeams. This secondary bow, a larger, fainter, optical echo of the first, is a chromatic mirror image: its colours are reversed, with red inside and violet on the outer edge.
