A friend of my daughter's snapped this Brocken Spectre near Abergavenny, in the Bannau Brycheiniog (Brecon Beacons).
When the sun shines from behind an observer onto a bank of mist, the observer's shadow may be cast on the mist and depending on the elevation of the sun the shadow may look like a giant figure, giving rise to the legend of the 'Brocken Spectre', named after the Brocken, a peak in the Harz mountains of northern Germany, where the condition are often favourable for seeing the phenomenon.
In this example the shadow appears tiny but we can clearly see the other feature of a Brocken Spectre: the observer's head is surrounded by a 'glory' – a ring of light like a halo. The ring has colours, in the same order as a rainbow, although distributed somewhat differently, but a glory is much smaller that a rainbow and caused by different mechanisms.
The water droplets in the mist are much smaller than the raindrops responsible for a rainbow, by a factor of about 100. The incoming light is back scattered and the colours are caused by the light waves interfering. But how does this back scattering happen?
Theory 1
The water droplets refract the light back. Unfortunately the water does not bend the light enough for this to be the explanation.
Theory 2
The physics of electromagnetic waves predicts the light can become a wave that travels round the surface of the raindrop and then emerges as a back scattered wave. This seems to happen but does not account for the amount of light actually back scattered.
Theory 3
More recent calculations have shown that energy can tunnel into a water droplet from light waves close by and after travelling round inside the droplet emerge as back scattered light, and that this effect can explain the observed brightness of glories. This is all explained in more detail in the excellent 2012 Scientific American article The Science of the Glory by H. Moysés Nussenzveig.
