雨過天晴，天邊時常會出現一道彩虹，給人帶來無限的遐想。有人說，彩虹是人間通往天堂的路，有人說，彩虹是女媧補天時留下的彩石。彩虹到底是怎么形成的呢？就讓下面這篇文章來為你揭開謎底吧！

The rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of about 40°-42°. This angle is independent of the size of the drop, but does depend on its refractive index. As seawater has a higher refractive index than rain water, the radius of a rainbow in a sea spray is smaller than that of a true rainbow. This is visible to the naked eye by a misalignment of these bows.

Since the water is dispersive, the amount that the sunlight is bent depends upon the wavelength, and hence color, of the light's constituent parts. Blue light is refracted at a greater angle than red light, but because the area of the back of the droplet has a focal point inside the droplet, the spectrum crosses itself, and therefore the red light appears higher in the sky, and forms the outer color of the rainbow. Contrary to popular belief, the light at the back of the raindrop does not undergo total internal reflection; however, light that emerges from the back of the raindrop does not create a rainbow between the observer and the sun. The spectra emitted from the back of the raindrop do not have a maximum of intensity, as the other visible rainbows do, and thus the colors blend together and do not form a rainbow.

A rainbow does not actually exist at a location in the sky, but rather is an optical phenomenon whose apparent position depends on the observer's location. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. These raindrops are perceived to constitute the rainbow by that observer. The position of a rainbow in the sky is always in the opposite direction of the Sun with respect to the observer, and the interior is always slightly brighter than the exterior. The bow is centered on the shadow of the observer's head, or more exactly at the antisolar point (which is below the horizon during the daytime), appearing at an angle of approximately 40°-42° to the line between the observer's head and its shadow. As a result, if the sun is higher than 42°, then the rainbow is below the horizon and cannot be seen as there are usually not enough raindrops between the horizon (that is: eye height) and the ground, to contribute. One exception is when the observer is at the top of a mountain or a similar vantage point, for example an aeroplane (see below). Another exception occurs when the rainbow is produced by a garden sprinkler. Although in this case to get sufficient drops they must be very small, resulting in a quite colorless bow.

dispersion: 散射

spherical: 球面的，球狀的

radius: 半徑

（通訊員稿 英語點津 Annabel 編輯）