When an observer looks up into the sky and sees an atmospheric rainbow they are looking at tiny images of the Sun mirrored in millions of individual raindrops. This is what produces the impression of arching bands of colour.
- It is the mirror-like surfaces on the inside of raindrops that reflect microscopic images of the Sun towards an observer.
- The images are tiny because raindrops are small, but also because the surface they reflect off is concave.
- At a micro-scale, each image of the Sun is different:
- Each and every image is a different colour and depends on the wavelength of light each raindrop is reflecting towards an observer’s eyes at any particular moment.
- For convenience sake, wavelength is usually measured in nanometres, but nanometres can be divided into picometres (or even smaller units). This means that an observer is looking at countless wavelengths of light and so countless colours.
- The images range in size and shape depending on the dimensions of the droplets and turbulence in the atmosphere. The size and roundness of raindrops also affect the appearance of a rainbow as a whole.
- The millions of microscopic images of the Sun that produce the impression of a rainbow is similar to the way pixels of light produce the images we see on digital displays.
- If all the rays of incident light that contribute to the formation of an observer’s rainbow are traced back from each raindrop towards the Sun it transpires that they are produced by parallel rays and that each incident ray is polarized as it passes through a droplet.
- If all the rays of incident light that travel through a single raindrop as it falls are compared, it transpires that they are all parallel with the axis of the rainbow.