The colour of water - a photo on Flickriver

            Water is very transparent in the blue but the absorption increases towards the red end of the spectrum due to vibrational transitions in the water molecule, H_2O. The swimming pool in the photograph is 2.1m deep and the sides and bottom are a light grey colour (almost white). The blue water in the left-hand image is due to this intrinsic absorption of red light.
In the normal visible colour image on the left, the water is very transparent and you see clearly to the bottom. The light returning from the bottom to your eye dominates over the image of the wall and trees reflected from the surface of the water (although you can still see them weakly)
The image on the right is taken with my Sony DSC-F717 camera in 'night-shot' mode with an RG780 IR filter. This has a bandpass shown in the spectrum above as 'Bandpass_RG780' which is the faint brown dashed line which peaks close to 800nm (the dashed red curve is the bandpass for an alternative filter, RG715, which accepts shorter wavelength light). This plot also shows the transmission of 5.5cm depth of water from my own observation (black line) and two papers published by: D. J. Segelstein, "The complex refractive index of water," University of Missouri-Kansas City, 1981 and by G. M. Hale and M. R. Querry, "Optical constants of water in the 200nm to 200µm wavelength region," Appl. Opt., 12, 555-563, 1973 (dark and light blue lines respectively - but note that their measurements were for highly pure water). At 800nm, the water absorbs a significant amount of light: 10% for just a few centimetres of depth.
This means that essentially no IR light reaches the bottom of the pool and so the water looks black: it is an almost perfect absorber. With no light emerging from the pool, the reflections from the surface are much more prominent: hence you see the wall and trees clearly - as well as the two leaves floating on the surface.
Water is extremely opaque over almost the entire electromagnetic spectrum from X-rays through to metre-length radio waves. The only exception is what we call the 'water hole' in the visible part of the spectrum where the transparency is very high. This remarkable property of water is crucial for the delevopment of photosynthetic life on the Earth since, without it, sunlight could not reach below the water surface or, indeed, even into water-containing plant leaves. The bright foliage at this wavelength contributes to the visibility of the reflection.
Since the main photosynthetic action of chlorophyll uses light at around 680nm, seaweeds growing at depths below a few metres need the presence of 'auxilliary pigments' to harvest bluer light for them. This is why deep-growing seaweeds are usually brown or reddish due to the addition of pigments such as phycoerythrin.
   

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Taken in: Italia / Umbria / Castel del Piano (show map)

Taken on: June 10, 2011

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Sets appears in: • Spectroscopy • Water more » • Natural Phenomena • The colour blue • Villa Aureli • Umbria • Nature slideshow • Cuthbert's: The Colour Blue « less

Groups appears in: • Earth Science Teaching Resource • Eye for Science more » • The Physics Classroom Flickr Group • Natural Phenomena « less

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