accommodation - The process of increasing the refractive power of the lens of the eye, focusing at a particular point within the depth of field, achieved by appropriate contraction or relaxation of the ciliary muscle which adjusts the shape of the lens; when the ciliary muscle contracts, the lens thickens, increasing its ability to focus at near objects.

photoreception - The detection, absorption, and use of light; for vision, special photoreceptor cells (rods and cones) convert light energy into nerve impulses.

retinal - The light absorbing lipid molecule derived from vitamin A, which can combine with proteins called opsins to forms four types of photosensitive molecules, the visual pigments.

opsins - Special photosensitive proteins which combine with retinal in cone cells to form a complex, a visual pigment, which will react with a portion of the spectrum of visible light by decomposing to initiate a series of chemical steps which lead to the generation of nerve impulses.

rhodopsin - The visual pigment (a lipid-protein complex) highly sensitive to red light located in the rods of the retinas of the eyes, consisting of an opsin and retinene which will react with a portion of the spectrum of visible light by decomposing to initiate a series of chemical steps which lead to the generation of nerve impulses.

light transduction - The conversion of kinetic light energy into the kinetic electrochemical energy of nerve impulses, i.e., action potentials, which is carried out by special photoreceptor cells; in vertebrates, this process occurs in the rods and cones of the retina of the eye.

light adaptation - Two processes, one involving constriction of the pupil (a pupillary reflex), by which the eye adapts to an increase in illumination, and the other, a response of the neurons of the retina to changes in the level of illumination such that the ratios of light intensities coming from different parts of the visual field remain constant, i.e., the contrast between lighter and darker objects is maintained.

dark adaptation - The physical and chemical adjustments of the eye, including dilation of the pupil  (a pupillary reflex) and increased activity of rods in the retina, that make vision possible in relative darkness with an increase in visual sensitivity developing with increasing time in the dark following exposure to light.

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