A shoal of tiny fish sheltering in the shade provided by the stern of a fishing boat anchored slightly upstream of the confluence of the Gosthani River.
Water is actually an opaque substance that absorbs both very long wavelengths such as Infra-Red - (700nm - 1 mm), and which makes up more than fifty percent of the energy of Sunlight - and very short wavelengths below 200nm.
It is transparent to the wavelengths between 200 nm and about 700 nm, that encompasses both visible light and the invisible Ultraviolet Light. And it is transparent to them because of a failure of quantum energy packet matching between the electronic, vibrational and rotational energy levels of the water molecule and the quantum packets of energy carried by these particular wavelengths. Not able to absorb these wavelengths into its electronic, vibrational or rotational energy levels, water, in a sense, “allows” these wavelengths to pass through - hence the transparency !!!. Water is clear, transparent, because of a failure!!!.
Ultraviolet light, particularly UV-B (290-320nm), is very damaging to cells, and water is especially transparent to it, only about 50% of UV-B being absorbed by the water even at 30 feet depth.
Fish living in deep lakes or deep rivers can go to deep depths to escape this fracturing photonic flood. Not so here given the shallowness of the stream. These fish therefore are staying out of the light for good reason, since UV-B can cause extensive DNA damage such as single strand breaks and Thymine dimer formation, not to mention free radicals and superoxide that are damaging to the cell in general. Indeed a case can be made that the nuclear envelope of the nucleus arose in Evolution to shield the chromosomal DNA from this fracturing flood of photons in the Proterozoic Era.
Being in the shelter of the stern might also prevent them from being swept downstream in the river current.
The Gosthani River. May 10, 2010. Bheemili. Andhra Pradesh.