芭樂熊熊

A rainbow is an optical and meteorological phenomenon that causes a nearly continuous spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc, with red on the outside and violet on the inside. More rarely, a double rainbow is seen, which includes a second, fainter arc with colours in the opposite order, that is, with violet on the outside and red on the inside.

Even though a rainbow spans a continuous spectrum of colours, traditionally the full sequence of colours is most commonly cited and remembered as red, orange, yellow, green, blue, indigo and violet. ("Roy G. Biv" and "Richard Of York Gave Battle In Vain" are popular mnemonics.)

Occasionally, a second, dimmer, and thicker secondary rainbow is seen outside the primary bow. Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50°–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.

「虹」形成原因是光線以一定角度照射在大氣中無數的水滴，再經過折射、分光、內反射、再折射等光學過程所造成的大氣現象。此現象若由太陽光線產生者，顏色鮮明；由月亮光產生者，則色澤黯淡。如圖一中所示，當太陽光線從Ａ點進入水滴內，陽光中各色光的折射程度不一樣，紫色光波長最短，其折射程度最大；紅色光波長較長，其折射程度最小。其餘各色則介乎其間，因此形成了一條內紫外紅的光圈，所以我們看到的是內紫外紅的彩色光帶，它的視角(從地面至虹頂的角度)約為42度。

「霓」亦稱副虹，和「虹」的不同僅在於形成過程中光線在雨滴內產生二次反射。因此，光線通過雨滴後射到我們眼內時，光弧彩色排列與虹正好相反，即是內紅外紫的彩色光帶。霓視角約為50度，與正虹為同心圓弧。偶而大氣中會出現三次虹及更高次虹，但因光度低極為罕見。

虹與霓的色彩及寬度，與雨滴的大小有關。雨滴越大，虹帶越窄，色彩也會更鮮明。相反來說，雨滴越小，虹帶越寬，色彩就比較黯淡。但雨滴太小時，分光和反射不明顯，也不會出現虹。此外，如果太陽的角度太大（例如在中午前後），或太小（近日出或日落）也不易看到虹。又因虹是陽光經雨滴反射進入觀察者的緣故，所以永遠出現在太陽的對面，因此朝虹見於西方，夕虹則見於東方。