IN HIS poem of 1820 entitled Lamia, John Keats complained that cold philosophy had destroyed the mystery of nature, and that Newton, through his work on optics, had “unweave[d the] rainbow”. Such a sentiment would find little sympathy with most scientists – or with most artists today for that matter. Indeed, an understanding of natural phenomena can only enhance our appreciation of nature and art.
Although it has long been known that a rainbow is produced by the dispersion of white light through rain droplets via refraction, there is far more to this optical phenomenon than first meets the eye. This interaction between light and water droplets can also create the “fog-bow”, the “dew-bow” and the “glory”, the details of which require some quite subtle physics to explain.
Despite being a familiar sight, rainbows are much harder to understand than one might think.The ingredients are, of course, sunlight and rain droplets. Although the Sun’s rays that reach the Earth are essentially parallel, the light impinges on a spherical droplet at a wide range of angles to the surface, where it undergoes refraction. When the light reaches the back of the droplet, two things can happen. The light can either refract and continue in a forward direction out of the drop, or it can be reflected internally, before passing back out through the front surface of the droplet via another refraction. It is this refracted and reflected light that creates the rainbow, which explains why rainbows only appear when one looks away from the Sun into a rain shower.
There are, however, inumerable raindrops at many different heights and positions above the horizon. As a result – and because of the many different angles at which the sunlight strikes the droplets’ surfaces – we receive light rays at many different angles to the “antisolar direction”, which is the direction looking away from the Sun towards the shadow of our head. So why does the bright, coloured arc of the rainbow only appear centred on this direction and at a specific and narrow range of angles to it?
In the February issue of Physics World John Hardwick, manager of the lightning division of Culham Electromagnetics and Lightning, in the UK unveils the subtlety of rainbows in more detail.