Casting shadows on the eye to reveal the inverted retinal image

Abstract

ABSTRACT We introduce and discuss an entoptic effect that helps to demonstrate that images on the retina are optically inverted and reversed. This simple consequence of geometric optics can seem somewhat abstract to students, but this effect is vivid, surprising, and links optical principles to students’ own subjective experience of seeing. Moreover, it requires nothing more than a bright light source, an opaque card that can be punctured to create a pinhole, and a pencil, pen, or other instrument with a fine tip. The phenomenon is not new, but we present it here in the hopes that it becomes more widely known and used in perception science instruction. We describe the necessary setup to achieve the effect, key observations, and briefly outline an explanation. We also contextualize the effect in historical discussion and observations of the inverted retinal image, including discussion of the pedagogical use of pinhole cameras in vision science instruction. Overall, this effect provides a simple yet dramatic demonstration of basic principles of image formation in the human eye.