The behavior of light under water is of great interest to photographers---and to that great majority of aquatic organisms that depend on solar energy for survival. Light entering water may be reflected, absorbed, scattered or transmitted. In lakes and oceans, the top layer of water through which light passes is called the photic zone. Its upper portion is the euphotic zone, where enough light enters to facilitate photosynthesis and where most aquatic organisms live.
The angle at which light hits the water's surface is also important. Penetration is greatest at midday, when the sun lies directly overhead. The lower angle of sunlight associated with morning and afternoon is more easily reflected. In deeper water, this means that the lower ranks of the photic zone may see a little weak light very briefly in the course of a day.
Light more easily passes through calm water. A roiled surface reflects more light. A northern lake, sealed by snow-carpeted ice, is almost wholly reflective.
Particles suspended in water will scatter light and shift visible color. Pure water tends to transmit longer blue wavelengths and absorb shorter ones, especially red; a deep blue can often signify nutrient-poor water (like some pelagic or open ocean regions), where there is little to scatter light and reveal hues of shorter wavelength. A lake rich with algae, by contrast, may be greenish because chlorophyll pigments in the phytoplankton absorb blue and red light.
Water is 800 times denser than air. Light entering water is refracted, or bent, at the surface, a phenomenon familiar to anyone who has observed the common demonstration of half-submerging a pencil in a beaker full of water. Objects under water also appear larger, as water's refractive index---a measure of how much a medium bends light---is 1 1/3 greater than air's.