The Brocken spectre (or Brocken bow) is an apparently greatly magnified shadow of an observer cast against mist or cloud below the level of a summit or ridge and surrounded by rainbow coloured fringes resulting from the diffraction of light. The effect is an illusion. Depth perception is altered by the
mist, causing the shadow to appear more distant and to be interpreted as larger than normally expected.
Actually the Brocken Spectre is what meteorologists call a glory. Most air travellers have already observed glories. They are most easily seen when one is riding on the shadow side of an aircraft above the clouds.
A Brocken Spectre can only be seen when specific conditions are met: The sun must be directly at one's back and there must be many suspended water droplets in the air where the Spectre's glory appears. Sunlight enters the water droplets and reflects off the back of the droplets and the light comes back towards the sun and the observer. This phenomenon is called diffraction and causes the circular rainbow-like bands around the shadow. Even if you are in a group you can only see your own shadow and glory (rainbow) ring, or your own Brocken Spectre. Therefore Brocken Spectres are confined to high-mountain areas when the sun is low.
Legend has that the name came from the Brocken, the highest peak of the Northern German
Harz Mountains. Once a climber was startled by the sudden appearance in the nearby mist of a human figure with a ring of light around its head. Frightened, the climber fell to his death, killed by his own shadow that he saw and the ring of light was his own glory ring.