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Perché il mare è blu ?
Skip to content Il colore blu del mare non dipende assolutamente dal colore del cielo. Molti tendono a pensare infatti che l’acqua del mare rifletta l’azzurro del cielo e ne dia cosi il suo colore. Niente di più sbagliato. Infatti la riflessione del colore del cielo è molto variabile ed incide in minima parte. E’ invece importante la diffusione e l’assorbimento della luce , soprattutto le lunghezze dell’onda minori dello spettro del visibile (dai 400 ai 550 nanometri circa). Acque limpide e calme hanno spesso un colore tendente all’indaco, azzurro o verde . Quest’ultimo colore può essere anche legato alla presenza di fitoplancton .
Lo scienziato Chandrasekhara Venkata Raman
Tra i primi a porsi la domanda perché il mare è blu fu il fisico indiano Chandrasekhara Venkata Raman . Lo scienziato cominciò a riflette durante una traversata notando che sebbene il cielo si scurisse perché in arrivo un temporale il mare rimanesse di colore blu.
L’assorbimento La luce solare è costituita da onde elettromagnetiche che danno vita allo spettro elettromagnetico che si suddivide in : raggi gamma, raggi x, ultravioletti, infrarossi, microonde e radioonde . Tra questi vi è lo spettro del visibil e con lunghezze d’onda inferiori all’infrarosso, comprese tra 400 e 750 nm(nanometri). L’occhio umano riesce a percepire i colori che vanno dal rosso all’arancione, passando per il giallo, verde, blu, indaco e violetto . I primi colori a scomparire in acqua sono rosso, arancione, giallo, gli altri scompaiono poi man mano che si scende in profondità. L’ ultima ad essere assorbita tra queste è appunto il blu. Questa lunghezza d’onda può anche raggiungere i 400 metri di profondità in caso di acque limpidissime dando al mare il caratteristico colore blu.
L’intensità della luce decresce in modo logaritmico in base alla profondità.
Perche l’acqua
che beviamo non è blu L’acqua in bottiglia o nel bicchiere non è blu perché le molecole di acqua presenti non sono sufficienti ad assorbire la luce. In mare invece accade che si diffonda un colore blu sempre più scuro più si va in profondità.
Maree rosse o verdi Capita sia in acque salate che in acque dolci che l’acqua di tinga di rosso o di verde in superficie. Spesso si tratta di bloom fitoplanctonici, delle fioriture algali massive. I bloom fitoplanctonici avvengono se ad esempio aumenta la disponibilità di nutrienti come azoto e fosforo. Questo accade se tali nutrienti arrivano dall’esterno o se si tratta di una risalita dal fondo di masse d’acque ricche di nutrienti, (fenomeno di UpWelling).
La penetrazione della luce in mare La luce in mare ha una grande influenza sulla vita degli
organismi marini. Infatti i raggi del sole più sono luminosi più penetrano in
profondità. Questo accade ogni giorno quando il sole si trova allo zenith. Nei
mari del Nord si ha una penetrazione della luce minore dovuta anche alla presenza di nubi e nebbie
che intercettano i raggi e per una minor trasparenza dell’acqua, dovuta ad una
grande sospensione di plancton.
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