Spirit in the sky: Hill walker captures ghostly figure at centre of rainbow 'halo' on Lake District fell in rare weather phenomenon known as 'Brocken Spectre'

 

The Brocken Spectre phenomenon is an optical illusion created when low sun shines behind someone looking down into fog from a ridge.

The atmospheric conditions throw the person's shadow forward, creating this spectral apparition.

It can appear on any misty mountainside or cloud bank, or even on some occasions, from an aeroplane.

The extraordinary sight involves the light behind a climber casting their shadow - often in an odd triangular shape.

The shadow can also fall on water droplets of varying distances from the observer's eye, causing confused depth perception.  

The Brocken Spectre got its name because of early sightings on the Brocken, the highest peak of Germany's Harz Mountains.

German scientist Johann Silberschlag first observed the phenomenon in 1780 but since then they have been seen and recorded many times in the region.

Among mountain climbers, there is a superstition that whoever sees a Brocken Spectre will die in the mountains the very next day.

Amazingly, the ghostly figure can sometimes appear to move rapidly when the cloud layer moves or there are variations in its density.

The head of the figure is often surrounded by rings of coloured light caused by light diffraction.

Like with a rainbow, this is caused by light refracting through tiny droplets of water in the air.

The phenomenon of refraction, as illustrated on Pink Floyd's Dark Side of the Moon album cover (right), occurs when light rays are bent as they pass through different transparent materials - in this case water and air.

When a beam crosses between the two materials the wavelength of the light changes.

As light is made up of several colours, when it is refracted these colours can bend at different amounts and become individually visible.

The Pink Floyd cover shows a triangular glass prism with a beam of light moving through the two sides causing the different colours to bend and break out in a rainbow.

Water in the air acts as tiny prisms so when light enters the raindrop, it reflects off the sides of the drop and is broken into a spectrum.

Another example of refraction would be a drinking straw in glass of water. When viewed from the side the straw appears to be broken at the point it enters the water due to the difference in the densities of water and air.