Slate & Rainbow Glints ~ Water drops bouncing off quarried slate glint rainbow colours. Imaged by Graham A Stephen (optical phenomena) at the National Slate Museum, Llanberis, Wales. ©Graham A Stephen, shown with permission.
The bow's centre is off the picture to the right. To the left, at the bow�s outer rim, water drops glint red light. Further leftwards they glint no light towards the camera at all � This is Alexander�s Dark Band. Contrast this with the drops to the right. They glint white light and brighten the inside of rainbowS which are disks rather than a rings of light.

Drops glinting a particular colour near the rainbow's rim 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.

It is not altogether that simple. Droplets deflect the sun's rays through a range of angles.

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 (except extreme red) is therefore diluted with some light of longer wavelengths.   Look carefully at the individual drops and you will see a few dimmer ones glinting other colours. Together these other colours add up to form the white light inside a bow.


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Cleaved and cut slate.

Slate is a metamorphic rock formed when shale consisting of clay minerals is heated under pressure to a few hundred degrees. Some clay minerals convert to mica and the rock develops a particular cleavage direction, not necessarily that of the original clay bedding planes.

In skilled hands, mined slate can be quickly split into thin sheets to form roofing tiles. Enormous deposits of slate existed In North Wales and it was long hand mined and cut. In the 18th Century slate mining became a major industry and literally and laboriously provided a roof for the UK.