Sunlight makes the first surprise visual, not chemical: from above, a glass-clear lagoon can flare turquoise where its surface receives light, its depth edits that light, and a pale floor returns what remains. No pigment is required. The eye reads an optical bargain, struck among water, sky, seabed, and the angle of view.
The mechanism is selective absorption. Water molecules absorb longer red and infrared wavelengths more readily than blue ones, so a longer optical path leaves a bluer signal; this is absorption spectroscopy at work, not a camera trick. Depth matters. In shallow water, photons that survive the trip can strike sand, coral rubble, or limestone, then scatter upward through the water column. That bright bottom reflection mixes with blue transmission, often producing turquoise rather than the darker blue of deep water.
Turquoise is a condition, not a permanent property. Suspended calcium carbonate can boost scattering, while algae or dissolved organic matter may push the hue toward green or brown; surface glare and skylight reflection can also mislead an aerial observer. Context wins. A pool with identical water can look blue, green, or nearly black as depth, bottom albedo, turbidity, and viewing geometry change. From overhead, color is less a label than light keeping accounts.
