RAINBOWS

DEFINITION: an arc or ring containing the colours of the spectrum in consecutive bands, formed in the sky by the refraction, reflection, and dispersion of light in rain or fog.

When light from a distant source, such as the sun, strikes a collection of water drops such as rain, spray, or fog a rainbow may appear. It appears as a multicoloured arc whose "ends" seem to touch the Earth. Rainbows are seen only when the observer is between the sun and the water drops, so rainbows appear in the part of the sky opposite the sun.

The centre of the rainbow's arc is located on an imaginary line extending from the light source through the observer's eye to the area of the water drops.

Rainbows are most commonly seen when the sun's rays strike raindrops falling from distant rain clouds. Generally, this is only in the early morning or late afternoon. When the sun is too far above the horizon no rainbow can be seen. When the sun is lower in the sky, however, part of the arc becomes visible. In fact, if the sun is low enough and the observer is located in a place that is high enough, such as on a mountain, in an aeroplane or a spaceship, the observer may see a circular rainbow.

The most brilliant and most commonly seen rainbow is called the primary rainbow. The arcs of colour in a rainbow are caused by the refraction, or bending, and internal reflection of light rays that enter the raindrops. A ray of white sunlight is actually composed of all the colours of the spectrum. Inside the drop the ray of white light is separated into the colours that make it up and reflected back toward the observer. In the primary rainbow the colours are, from inside to outside, violet, blue, green, yellow, orange, and red. The red band makes an angle of about 42 degrees with the sun's rays, and the other coloured bands make successively smaller angles. Sometimes another less intense rainbow may also be seen; this is called the secondary bow. The secondary bow, when visible, is seen outside the primary bow and with its colour sequence reversed. It is produced by light that has been reflected from two different points on the back of the drop before emerging into the air. Higher-order rainbows are very weak and so are rarely seen.

Occasionally, faintly coloured rings are seen just inside the primary bow. These are called spurious, or supernumerary, bows. When raindrops are extremely fine, an almost white bow, called a fog bow, is produced. A fog bow at night, sometimes called a lunar rainbow, is made by sunlight reflected from the moon and appears as a ring around the moon.