Another Glimpse Of A Rainbow
To see a rainbow, the viewer must have the sun behind them at an elevation of less than forty-two degrees, the point at which shadow’s length becomes greater than their height. There must be water droplets in front of them, usually in the form of a passing rain shower, falling in the direction of 42 degrees from their shadow. If the sun’s elevation is greater than forty-two degrees, the rainbow is out of sight below the horizon. The lower the sun’s elevation, the taller the rainbow.
Intriguingly, a rainbow is little more than an optical illusion; it does not exist in a specific spot in the sky. What this means is that even if two people standing side by side see a rainbow in the sky, it is unique to each observer. It also raises the deep philosophical question of whether a rainbow really exists without the presence of a sentient being to recognise it for what it is.
Is there is a pot of gold buried at the end of the arc? The origins of this enduring myth lie in an Irish folktale about the Viking’s invasion of their island. Looting and plundering at will, they buried their spoils all around the countryside. In their haste to leave, they left some of their treasure behind, which the leprechauns found. Being distrustful of humans, the leprechauns took the gold and buried it deep underground around the island, the only clue to its whereabouts being the end of a rainbow.
Anyone hoping to get rich quick by unearthing a pot of gold will be sorely disappointed. As raindrops are spherical, when sunlight passes through them at just the right angle, they form a circular rainbow. The reason that only a part of a rainbow, the arc, is usually visible is that the Earth’s surface blocks out the rest of the light. A rainbow in its full circular glory can occasionally be seen from the top of a mountain or the cockpit of an aeroplane.
Looking out of an aeroplane window, a passenger might see what looks like a circular rainbow around the shadow of their head, made up of one or more successively dimmer concentric circles, each of which has red on the outside and blue on the inside. Like a rainbow it is an optical illusion, known as a glory, but is the result of a completely different physical process, the wave interference of light, usually from the sun but also the moon, internally refracted within small droplets.
Occasionally a second rainbow appears in the sky slightly above the primary rainbow. It develops when light entering a raindrop undergoes two reflections instead of one and is scattered at an angle of fifty-one degrees. The second reflection not only reduces the intensity of the light but also reverses the order of the colours, with blue at the top and red at the bottom.
A tertiary rainbow occurs when light is reflected for a third time. The these can only be seen by looking directly towards the sun, because the sun rather than the point directly opposite (the antisolar point) is its centre. This makes it difficult to spot, especially as the intensity of the colours is much reduced, and the bands are broader.
A quaternary rainbow, where the light is reflected four times, is fainter still and can only be observed while looking at the sun. Under laboratory conditions, scientists had detected a 200th-order rainbow, where the light has been reflected two hundred times.
Whether you are of a scientific or romantic disposition, there is much to marvel about a rainbow. As William Wordsworth observed; “my heart leaps up when I behold/ a rainbow in the sky…”
