The surface cheats the spectator. A fish seen from above can appear closer to the surface than it really is, because light changes direction as it passes from water into air. Depth becomes a visual fiction. Beneath that apparently accessible target lies a mismatch between where light places the fish and where its body swims.
The deception needs no murk. Even clear, still water produces this displacement, since its refractive index differs from that of the air through which the observer receives the light. Refraction does the work. As rays leave water at an angle, they bend away from the normal, the imaginary line perpendicular to the surface at the crossing point. Sight supplies the shortcut. The visual system interprets those arriving rays as though they had followed straight paths, placing their apparent source above the fish's actual position. Geometry defeats first impressions. Snell's law describes the relationship between the ray angles and the two materials' refractive indices.
Clarity is not positional honesty. For viewing close to directly overhead through a flat surface, apparent depth is approximately three quarters of actual depth, using the usual refractive indices of water and air. Angles complicate that estimate. A more slanted view changes the apparent position, while ripples introduce shifting surface angles that can make the image wobble or break apart. The fish remains below. For the spotted spectator, a visible outline therefore offers evidence of a fish without delivering an exact address, however confidently the eye seems to locate it. The surface keeps its secret.
