A "rainbow-bridge" devised by Oromė for Manwė and the Valar. The mythical vision for the Rainbow, the Bridge of Heaven to be used by those Valar who would wish to visit the Great Lands (Middle-eart) and come back to Valinor after the Blissful Realm had been hidden. Few Eldar ever had the hearts to pass this bridge. But for the Elves and Men other road was set - Qalvanda.(as per BoLT1). It is only visible after rain, when it is wet.
|Then said he (Oromė): "Lo, O Sślimo Lord of the Airs, a way I have devised whereby any of the Valar of good heart may fare whithersoever they list in the Great Lands; for whither they wish I will cast my slender bridge, and its hither end wilt thou securely guard."
And of this work of Oromė's came that mighty wonder of the heavens that all men look upon and marvel at, and some fear much, pondering what it may portend. Yet doth that bridge wear a different aspect at different times and in various regions of the Earth, and seldom is it visible to Men and Elves. Now because it glistens most marvellously in the slanting rays of the Sun, and when the rains of heaven moisten it it shines most magically therein and the gold light breaks upon its dripping cords to many hues of purple, green, and red, so do men most often name it the Rainbow, but many other names have they fashioned also, and the fairies call it Ilweran the Bridge of Heaven.|
|HoMeI - "The Hiding of Valinor"|