THE RAINBOW

Evening Star, Issue 21319, 25 January 1933, Page 9

THE RAINBOW

HOW IT IS CREATED LIGHT OF SUN AND MOON The meteorologist recognises a great variety of what are called “ optical phenomena,” all of them produced by the reflection, refraction, or diffraction of the light from the sun or mo9n by droplets of water or crystals of ice in the atmosphere. Among these rainbows are perhaps the most beautiful, and they are certainly the most familiar. Like many other familiar things, rainbows present features of. considerable scientific interest, and it is only recently that a complete explanation has been found for the peculiarities sometimes observed (says a writer in the ‘Manchester Guardian’). For a natural rainbow to be produced three conditions must be satisfied; first, rain must be falling near the observer in the direction opposite to that of the sun; secondly, the sun itself must not be obscured by cloud; thirdly (and this is an important point), the sun must not be too high in the heavens. Suppose the sun to be nearly setting on the western horizon, then a rainbow will be seen east of the observer if a shower is falling in that direction. The rainbow will appear as a semi-circular arch with its centre exactly on the prolongation ot a line drawn from the sun, to the observer’s head. The colours are those of the spectrum, ranging from violet on the inside, through blue, green, yel~ low, and orange, to red on the outside. The “size” of the bow is best indicated by the angle through, which it is necessary to turn a sighting rod or telescope in moving from the centre to the edge of the bow, and this angle is approximately 42deg. WHEN THE SUN IS HIGH.

If the sun is well above tbe horizon the rainbow will be seen, as a smaller arc of a circle instead of as a semicircle, because the centre of the arch is in all cases exactly opposite the centre of • the sun. The higher the sun the more will the rainbow appear to be depressed. The radius of the bow is 42deg; consequently if the sun is more than 42deg above the horizon no bow can be seen at all, unless the observer stands on a hill, in winch case he may see part ot the bow projected against low-fying fields, etc if ram is falling between him and them. In latitude 55deg t N., the mean latitude. ot the British Isles, the altitude of the sun exceeds 42deg in June from about 10 a.m. to 4 p.m. Consequently it is impossible for an observer standing on the ground to see a. rainbow m June between those hours. . On the south coast of England a rainboweaniiotbo observed at midday (1 o clock daylight saving time) between the end of March and &e middle of September. In the middle of August rainbows can be seen S the North of England up to about 11 a.m. and after about 3 p.m. At the hours mentioned the extreme top of the bow will just project above tne horizon. A rainbow of appreciable height would only bo seen before about 10.30 a.m., and after about 3.30 p.m. Obviously tbe best time to seeajainbow is near sunset; sunrise is less lav curable, because the required,.showery conditions are not often experienced at that hour. In late autumn, winter, and early spring it is possible to see a rainbow at any time ot day, but in those seasons the required showery conditions are less common, and brdliant bows are infrequent. IN THUNDER RAIN. The brilliance and colouring of a rainbow depend very largely on the size ot the raindrops in the shower. To produce a brilliant bow showing the full range of spectrum colours from violet to red the drops must exceed one-twenty-fifth of an inch m diameter, and these are normally found in showers of the thundery type. When thedropsare much, smaller than this tho led G1 4. 0 * the spectrum disappears, and the outer edge of the bow is coloured orange. These remarks refer to the rainbow most frequently seen—namely, the “ primary ’ ’ bow. When this is very brilliant another bow, the ‘‘secondary rainbow, is seen outside the primary. In this bow, which is usually much fainter than the primary, the colours are reversed, red being inside and blue outside. Sometimes what are known as “ supernumerary ” bows of a pinkish colour are seen inside the primary, near the top of the arch. The region of the sky outside the primary bow is noticeably brighter than the region within the arch. Outside the secondary bow the sky is rather brighter than, in the region between the two bows. The supernumerary hows are due to the scattering or diffraction of light by very small dropletfe. The apparent nearness or remoteness of a rainbow depends entirely* on the distance between the shower and the observer. In favourable circumstances a natural eainbow. bq geen. within

a hundred yards or less, and an artificial rainbow, produced by the spray from a fountain or garden hose, may be formed within a few feet of the observer. While it is possible in this way to see a rainbow at close quarters it is clearly impossible to walk through a rainbow.” In all cases the rainbow is an arc of a circle the centre of which is the shadow of the observer’s head and to walk through a rainbow is as impossible as to run away from your shadow. Rainbows may be formed by the light of the moon as well as by the light of the sun. The same conditions must be fulfilled, but it is not so easy for these conditions to occur by night as by day. To obtain sufficient illumination the moon must be nearly or quite full, and the necessary localised showers are not nearly so common during the night as during the day. Lunar rainbows: are therefore rather uncommon, and as the human eye cannot distinguish colours when the illumination is faint they appear to be white. . „ A “ fogbow ” may be seen in favourable circumstances when the sun shines from behind the observer on to a bank of fog. As the droplets in a fog are very minute very little colour is seen in a fogbow; all that can be distinguished is a bluish tinge on the inner edge and a reddish tinge on the outer edge. The phenomenon is most frequently seen from a hilltop, and the observer may at the same time observe his own shadow on the fog bank, with a “ glory ”, of coloured rings surrounding the head—the “ Brocken spectre.”