{"id":7709,"date":"2017-05-09T16:44:00","date_gmt":"2017-05-09T16:44:00","guid":{"rendered":"https:\/\/blogs.futura-sciences.com\/feldmann\/?p=7709"},"modified":"2017-05-21T20:04:28","modified_gmt":"2017-05-21T20:04:28","slug":"se-forme-double-arc-ciel","status":"publish","type":"post","link":"https:\/\/blogs.futura-sciences.com\/feldmann\/2017\/05\/09\/se-forme-double-arc-ciel\/","title":{"rendered":"Comment se forme un double arc-en-ciel ?"},"content":{"rendered":"

Un bel arc-en-ciel est parfois accompagn\u00e9 d’un second, moins lumineux et plus \u00e9tal\u00e9. Explications avec une image r\u00e9alis\u00e9e en Pologne.<\/strong><\/p>\n

De tous les photom\u00e9t\u00e9ores l\u2019arc-en-ciel est sans aucun doute le plus beau, et ce n’est pas ce clich\u00e9 r\u00e9alis\u00e9 le 13 avril en Pologne par Kamila Mazurkiewicz Osiak<\/a> qui nous fera mentir. En traversant les gouttes de pluie, les rayons du Soleil<\/a> (et parfois m\u00eame ceux de la Lune<\/a>) sont r\u00e9fract\u00e9s (leur trajectoire s’incurve) et d\u00e9compos\u00e9s en une succession de couleurs.<\/p>\n

\"\"<\/a><\/p>\n

La plupart du temps on observe un seul arc-en-ciel, appel\u00e9 arc primaire, qui se forme sur un cercle dont le centre correspond au point anti-solaire (un point \u00e0 l’oppos\u00e9 du Soleil par rapport \u00e0 l’observateur).\u00a0<\/p>\n

Mais il arrive que cet arc primaire, s’il est tr\u00e8s lumineux, s’accompagne d\u2019un arc secondaire dont l\u2019ordre des couleurs est invers\u00e9. Plus \u00e9tal\u00e9 et moins brillant (son \u00e9clat correspond \u00e0 environ 40% de celui de l’arc primaire), l\u2019arc secondaire est provoqu\u00e9 par la double r\u00e9flexion des rayons lumineux \u00e0 travers les gouttes de pluie qui se comportent comme des prismes.<\/p>\n

L’arc-en-ciel a toujours fascin\u00e9 les hommes : 300 ans avant J.-C. le philosophe grec Aristote avait d\u00e9j\u00e0 remarqu\u00e9 que le rayon moyen de l’arc primaire est de 41\u00b0 (51\u00b0 pour l’arc secondaire). Un peu plus tard c’est Alexandre d’Aphrodise qui nota que la zone comprise entre les deux arcs est plus sombre que le reste du ciel (en raison d’un ph\u00e9nom\u00e8ne d’interf\u00e9rence entre les rayons lumineux d\u00e9couvert par le physicien britannique George Airy), zone d\u00e9sormais appel\u00e9e bande d’Alexandre.<\/p>\n

Au XI\u00e8me si\u00e8cle la d\u00e9couverte de la r\u00e9fraction par le math\u00e9maticien et physicien arabe Alhazen a permis de comprendre le principe de ce photom\u00e9t\u00e9ore<\/a>. Plus tard le c\u00e9l\u00e8bre Isaac Newton a choisi arbitrairement de d\u00e9couper l\u2019arc-en-ciel en sept couleurs : rouge, orange, jaune, vert, bleu, indigo et violet (de l\u2019ext\u00e9rieur vers l\u2019int\u00e9rieur pour l’arc primaire).<\/p>\n

En Europe une tradition veut que l\u2019on trouve un chaudron rempli d\u2019or au pied de l\u2019arc-en-ciel.<\/p>\n","protected":false},"excerpt":{"rendered":"

Un bel arc-en-ciel est parfois accompagn\u00e9 d’un second, moins lumineux et plus \u00e9tal\u00e9. Explications avec une image r\u00e9alis\u00e9e en Pologne. De tous les photom\u00e9t\u00e9ores l\u2019arc-en-ciel est sans aucun doute le plus beau, et ce n’est pas ce clich\u00e9 r\u00e9alis\u00e9 le 13 avril en Pologne par Kamila Mazurkiewicz Osiak qui nous fera mentir. En traversant les … Continuer la lecture de Comment se forme un double arc-en-ciel ?<\/span> →<\/span><\/a><\/p>\n","protected":false},"author":8,"featured_media":7710,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_links_to":"","_links_to_target":""},"categories":[85],"tags":[2172,2174,262,2171,2173,696,2169,520,2170],"class_list":["post-7709","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-meteores","tag-alexandre-daphrodise","tag-alhazen","tag-arc-en-ciel","tag-aristote","tag-george-airy","tag-isaac-newton","tag-kamila-mazurkiewicz-osiak","tag-photometeore","tag-pologne"],"yoast_head":"\nComment se forme un double arc-en-ciel ?<\/title>\n<meta name=\"description\" content=\"Un bel arc-en-ciel est parfois accompagn\u00e9 d'un second, moins lumineux et plus \u00e9tal\u00e9. 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