|
The light will separate into its component colours as they travel at different speeds through the prism. This phenomenon is called refraction and can be witnessed in nature.
All that is required is a keen eye, an occasional upward glance...
There are, in fact, a large number of optical phenomena to be found in the skies, both by day and by night. You don't have to be a 'weather anorak' to see haloes, coronas, fogbows and mock suns. All that is required is a keen eye, an occasional upward glance and an awareness of what to look for.
Haloes
Haloes appear around the Sun or Moon when a layer of cloud composed of ice crystals lies between the Sun (or Moon) and the observer. Ice crystals act as tiny prisms and light is refracted to produce, most commonly, a 22 degree halo. The cloud most suited to halo formation is cirrostratus, which is most commonly observed well ahead of a developing frontal system. So a halo is a good guide to approaching bad weather.
Mock suns or sun dogs
Another type of ice crystal is responsible for the much rarer 46 degree halo. Haloes may be accompanied by a whole array of circumzenithal arcs and parhelion (mock suns). Mock suns, or sun dogs as they are also known, appear as bright spots on either side of the Sun. Moon dogs have been observed but they are very rare.
Coronas
Coronas are luminous disks that may appear around the Sun or Moon when viewed through a thin layer of cloud consisting of water droplets. As with the rainbow, this effect is caused by the bending of light. But the corona owes its origin to diffraction rather than refraction. A corona is best seen when the Moon acts as the source of light - the Sun tend to blind the observer to the relatively subtle colours produced by diffraction.
Rainbows
Rainbows are familiar to us all. They occur when sunlight falls upon raindrops, which act as tiny prisms. This causes the constituent colours of sunlight to leave at slightly different angles. The sun is always behind you as you face the rainbow. The higher the Sun the flatter the rainbow will be. When the Sun rises higher than 42 degrees above the horizon, the rainbow will disappear. Notice how the sky inside the arc appears brighter than the area outside. Watch out for faint arcs near the top of the rainbow. Lunar rainbows, cloudbows and fogbows are similar to rainbows but are generally much fainter.
Iridescence
Iridescence appears as irregular patches of colour within a cloud and owes its formation to the same processes which cause coronas. So even on the dullest, most overcast day there is always the chance than the lowest cloud layers will break, giving us a glimpse of something far more interesting. And if the cloud does close in then there is always the chance of seeing a fogbow and cloudbow.
Brocken Spectre
Another optical illusions that can be caused by light is the Brocken Spectre. This strange effect is often witnessed by climbers when the sun is low in the sky. The climbers shadows are projected as large figures on the clouds above them. The phenomenal size of these figures is partially due to the shadows being cast on to clouds, which are not solid objects. Therefore the depth of the shadow can be several metres rather than on one plane. This makes it appear much larger than it would normally be.
Mirages
Of all the Sun's optical illusions, the most famous has to be the mirage. This is caused by light being bent as it passes through different elements of the atmosphere that have different densities.
| 
