(a) Top, In a typical zebrafish neuroscience experiment, an image is presented via projection onto a screen underneath an animal in a water-filled plastic dish. Middle, A small layer of air separates the screen from the dish and water. Bottom box, This configuration causes the image received at the eye (brown arrow) to be distorted and translated relative to the projected image (black arrow). We can describe this transformation as a relationship between the true position of a projected point () and its apparent position (), depending on the ratio between the distance from the air-water interface to the screen () and the distance from the eye to the air-water interface (). To solve the transformation, we use Snell’s law (illustrated in inset and panel b), which relates the angle at which a light ray leaves the air-water interface () to the angle at which it hits the interface (), depending on the refractive indices of the media (air, ; water, ). Note that the effects of the plastic dish are typically minor (Appendix 1). (b) Top left, the apparent position of a point () as a function of its true position (), and its inverse (inset), for (pink) and (blue). Top right, fraction of light transmitted into the water as a function of for the same two values of . Bottom box, Using Snell’s law, we derived (top left inset), whose inverse we take numerically to arrive at (top left). (c) Simulated distortion of a standard sinusoidal grating. Yellow circle denotes the extent of the Snell window (~97.2° visual angle). The virtual screen is modeled as a 4 × 4 cm square with 250 pixels/cm resolution, and we fixed the total distance between the fish and the virtual screen, , to be 1 cm. Note that only a fraction of the screen is apparent when is small (bottom left), but a distorted view of the full screen appears within the Snell window when becomes large (bottom right). Contrast axes are matched across panels and saturate to de-emphasize the ring of light at the Snell window, whose magnitude would be attenuated by unmodeled optics in the fish eye (Materials and methods).