Rainbow Glints

Paul Gitto captured this close-up of a rainbow made by a lawn sprinkler.
ŠPaul Gitto

Images like this show that rainbows are the glints of coloured light from millions of individual water drops.

The primary bow is produced by rays that enter a drop, reflect off the back surface and leave the side of the drop that they entered. The two refractions at the entrance and exit disperse the light into colours.

Drops glinting a particular colour at the rainbow's edge towards the eye or camera are on or near the surface of a cone whose apex is at the eye and axis is parallel to the direction of sunlight - a 'rainbow cone'.

The red cone is widest, producing the red outer edge to the primary bow. The green and blue cones are progressively narrower.

In the image we see the glints of drops frozen in one posion but moving moving drops can successively glint a number of colours to the eye as it falls through each different colour rainbow cone.

It is not altogether that simple because droplets deflect the sun's rays through a range of angles. The colours at the rainbow rim are those from rays undergoing minimum deflection through the drops. There are greater deflection angles too.

If we think only of red rays, the red edge of the bow is produced by drops near the surface of the 'red cone'. Drops within the cone also direct red light to the eye and any colour of the rainbow is diluted with some light of longer wavelengths. Look carefully at the individual drops and you will see some glinting in other colours from the majority.

This is best seen by standing inside a sunlit sprinkler spray and getting very wet!
Atmospheric
Optics

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