• Question: why is the sea water blue and not clear like tap water?

    Asked by summer to Thomas on 22 Jun 2015.
    • Photo: Thomas Farrugia

      Thomas Farrugia answered on 22 Jun 2015:

      Hi Summer – this isn’t as straightforward as it seems i.e it’s not just a case of “the sky is blue therefore the sea is blue”. The main cause for the sea’s blue colour is the absorption of certain components of white light – in the case of water most of the red is absorbed in the first 15 metres or so (which is why things seem quite dull when you scuba-dive), whereas the blues and greens penetrate to much deeper depths. Any light that we see coming from the sea is going to be richer in the blue, and hence that’s the colour we see.

      Both a small amount of sea water and tap water are clear because there isn’t enough water to absorb enough of the red component in white light – but if you were to fill up a swimming pool with enough tap water you’d see the same effect.

      I hope this made sense, physics isn’t my exact domain, but this was a good question to explain 🙂

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