h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

F7180006?thread=6057643

This was a question I posed in the Quite Interesting society about placating anxious jellyfish.

The explanation hinges on a piece of research that showed bathing them in green light sedates them.

The explanation given was that green light is what their primitive light-sensing cells associate with the natural habitat depth.

Now I completely ballsed up the explanation, getting my long and short-wavelengths back-to-front and then compounding this by ascribing to the wrong wavelength the property intended.

However now an intellectual disagreement has broken out about why water is blue whether it is naturally blue (as was claimed in a recent episode of QI proper) or whether this phenomenon of blue at depth is also correct

And frankly I'm now so completely turned around on something I previously thought I'd understood I'm looking to the SExperts for some clarity on the matter.

Clive.

From what I can see from that conversation InfiniteImp was pretty much correct in what (s)he said.

Although water does absorb (very very slightly) in the red end of the visible spectrum which contributes significantly to its (very very slight) blue colour.

AFAIK the sky and water are blue because they both scatter blue (short wavelength) light and in the case of the sky it is water vapour that is the main culprit.

In the case of water - when you look at the surface of a body of water it's colour is affected by a number of things: -

1. The reflected colour of the sky;
2. the composition/colour of the underlying bed if the water is relatively shallow i.e. sand, rock, mud, weed etc;
3. the depth of the water;
4. the amount of suspended particulates and salinity.

1 and 2 are also affected by the angle at which you are looking on the water.

At depth the light gets bluer and dimmer until it becomes dark at about 200m.
http://science.jrank.org/pages/4836/Ocean-Zones-Water-depth-vs-light-penetration.html

t.

>>At depth the light gets bluer and dimmer until it becomes dark at about 200m.<<

That's it. the blue wavelength travels furthest but just above this region is a layer or depth where blue and green light is still present, and that is the jellyfishes' habitat.

Which is what I was trying to explain.

So scientifically is this sounding about right? (green light being a slightly longer wavelength than blue light gets scattered higher up in the water column, until, as you descend blue light predominates at depth but is dimmer due to it's being that last frequency to be scattered of the white light at the surface.

I'm still quite confused about this but is this accurate?

If it is just being scattered then it will still be there.

The further light penetrates through a substance the more it is absorbed.

It gets dark ultimately because of absorption in the end. Although the blue appearance of surface water is predominantly to do with scattering yes.

Red, then green, then blue will disappear by absorption in that order, the deeper you go.