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This story begins with the Sun and the nature of the radiation from it which falls on the Earth. The Sun puts out energy in the form of electromagnetic waves. These waves in electric and magnetic fields, like ocean waves, have a certain distance from peak to peak, called their wave length. Also like ocean waves a certain number of waves pass a fixed point per unit time. That number is called the frequency of the wave. Again like ocean waves if we multiply the wave length time the number (frequency) of waves arriving per unit time, we get the speed of the waves.

There are some significant differences between electromagnetic waves and ocean waves. The speed of ocean waves may be different for each other. The speed of electromagnetic waves is constant, regardless of frequency. That means that the wave length of electromagnetic waves is determined by their frequency. Also unlike ocean waves which deliver their energy in various amounts when they break on the shore, electromagnetic waves deliver a fixed amount of energy, proportional to the frequency, when they interact with matter. So it turns out that both the wave length and energy of electromagnetic waves depend on their frequencies.

The electromagnetic waves arriving at the Earth from the Sun are in trains of many different frequencies. When these waves fall on sea water, their interaction with the water depends on their wave length and energy, an therefore their frequency. The low frequency, long wave length waves and the high frequency short wave length waves are reflected or absorbed in the water. Only a small band of medium frequencies penetrate the ocean to any significant depth. As sea creatures evolve, those who could sense this narrow band of electromagnetic waves would be able to find food and avoid enemies better than animals sensitive to some other electromagnetic waves, or none at all. We should expect modern day sea creatures to have this sensitivity, and they do.

The coincidence is that land creatures including humans are also sensitive to this band of electromagnetic waves that penetrate sea water. We humans call this part of the electromagnetic spectrum... visible light.