sciences: La potion d’invisibilité lien chap 10 et 12 T1 Harry Potter
[lien cape d’invisibilité d’Harry, casque d’invisibilité de Persée; les potions]
Dans le cadre de notre étude sur les solutions,
29/04 On a fait une expérience proposée par Haëline le 11/03. Dans cette expérience, on utilise de la glycérine pour tenter de rendre un objet invisible.
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2/05 On a remplacé la glycérine par de l’eau pour voir la différence:
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Avec l’eau, on le voit beaucoup mieux l’objet. La glycérine rend l’objet plus difficile à voir.
Pourquoi la glycérine marche mieux que l’eau?
explication: La lumière se déplace à des vitesses différentes dans des matériaux différents. La glycérine a un indice de réfraction plus élevé que l’eau.
C’est quoi un indice de réfraction? C’est une manière de dire à quelle vitesse la lumière traverse une matière.
-si la lumière ralentit beaucoup, l’indice est élevé.
-si elle ralentit peu, l’indice est plus bas.
Si l’objet plongé dans un liquide a presque le même indice de réfraction que le liquide, alors:
-la lumière ne dévie pas beaucoup, et il n’y a presque pas de frontière visible
-On ne distingue plus les contours de l’objet
-Il devient presque invisible!
conclusion: La glycérine a un indice plus proche de certains plastiques transparents que l’eau. C’est pour ça que l’objet « disparait » mieux dans la glycérine que dans l’eau.
C’est comme de la vraie magie…mais c’est de la science!
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18/11